fix(insights): align cold-fallback slope window to the rollup day boundary

The rollup read path windows the 7/30/90-day regression on
startOfUtcDay(now - N days), while the live-SQL fallback filtered the
REGR_* aggregates on the wall-clock NOW() - INTERVAL bound. An account
crossing a coverage boundary (warm to cold) therefore saw the boundary
day's readings flip in or out of the slope window depending only on
whether the rollup tier was populated, returning a cache-dependent
slope for the same request.

Anchor both cold-fallback paths on date_trunc('day', NOW() AT TIME ZONE
'UTC') - INTERVAL 'N days', the SQL twin of startOfUtcDay, so warm and
cold return identical boundary-day membership. The avg7/avg30/median
windows stay wall-clock since the warm narrow query computes them the
same way.

src/lib/analytics/__tests__/summaries-slice.test.ts

@@ -652,4 +652,54 @@ describe("computeSummariesSlice", () => {
       );
     });
   });
+
+  /**
+   * A2-M2 — read-swap boundary-day consistency. The warm
+   * `computeFromRollups` path windows the regression on UTC-midnight
+   * (`startOfUtcDay(now − N days)` + `composeWindowedRegression`). The
+   * cold-fallback `windowed` query must anchor its REGR_* FILTERs on the
+   * SAME UTC-midnight boundary — `date_trunc('day', NOW() AT TIME ZONE
+   * 'UTC') − INTERVAL 'N days'` — so a warm→cold coverage transition
+   * returns identical boundary-day membership for slope7/30/90 instead of
+   * a cache-dependent answer to the same request.
+   */
+  describe("A2-M2 regression-window boundary alignment", () => {
+    it("anchors the cold-fallback slope FILTERs on the UTC-midnight day boundary, not the wall-clock instant", async () => {
+      const queries: string[] = [];
+      UNSAFE.mockImplementation((sql: string) => {
+        queries.push(sql);
+        return Promise.resolve([]);
+      });
+
+      // Empty coverage ⇒ cold-fallback path ⇒ the `windowed` query runs.
+      RAW.mockResolvedValueOnce([]);
+      await computeSummariesSlice("user-a2m2");
+
+      const windowedSql = queries.find(
+        (sql) => sql.includes("AS slope7") && sql.includes("REGR_SLOPE"),
+      );
+      expect(windowedSql).toBeDefined();
+      const sql = windowedSql as string;
+
+      // Every regression window (7/30/90 for slope + r²) anchors on the
+      // day-truncated UTC boundary. Six FILTERs total.
+      for (const days of ["7 days", "30 days", "90 days"]) {
+        expect(sql).toContain(
+          `(date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '${days}') AT TIME ZONE 'UTC'`,
+        );
+      }
+      const anchored = sql.match(
+        /date_trunc\('day', NOW\(\) AT TIME ZONE 'UTC'\) - INTERVAL '(?:7|30|90) days'\) AT TIME ZONE 'UTC'/g,
+      );
+      expect(anchored?.length).toBe(6);
+
+      // The regression windows must NOT fall back to the bare wall-clock
+      // `NOW() - INTERVAL 'N days'` bound the warm path never uses for the
+      // slope. Isolate the slope columns (everything from the first
+      // REGR_SLOPE onward) so the avg7/avg30 windows above — which DO stay
+      // wall-clock by design — don't trip the assertion.
+      const slopeBlock = sql.slice(sql.indexOf("REGR_SLOPE"));
+      expect(slopeBlock).not.toMatch(/WHERE m\."measured_at" >= NOW\(\) -/);
+    });
+  });
 });

src/lib/analytics/summaries-slice.ts

@@ -772,41 +772,54 @@ async function computeFromLiveAggregate(
         ) FILTER (
           WHERE m."measured_at" >= NOW() - INTERVAL '90 days'
         )::double precision                                           AS median,
+        -- A2-M2 — anchor the regression windows on the UTC-midnight day
+        -- boundary so this cold-fallback path returns the SAME boundary-day
+        -- membership the warm computeFromRollups composes via
+        -- startOfUtcDay(now - N days) + composeWindowedRegression. The
+        -- expression date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL
+        -- 'N days' is the SQL twin of startOfUtcDay; the trailing AT TIME
+        -- ZONE 'UTC' re-materialises the naive truncation as a UTC instant
+        -- so the measured_at (timestamptz) comparison lands on the same wall
+        -- the rollup bucketStart sits on. The wall-clock NOW() - INTERVAL
+        -- bound let the boundary day flip in/out of the slope window across
+        -- a warm-to-cold coverage transition (cache-dependent slope on the
+        -- same request); the avg7/30/median windows above stay wall-clock
+        -- because the warm narrow query computes THOSE the same way.
         REGR_SLOPE(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '7 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '7 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS slope7,
         REGR_R2(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '7 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '7 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS r2_7,
         REGR_SLOPE(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '30 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '30 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS slope30,
         REGR_R2(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '30 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '30 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS r2_30,
         REGR_SLOPE(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '90 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '90 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS slope90,
         REGR_R2(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '90 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '90 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS r2_90
       FROM ${canonicalMeasurementsFrom(rankUnqualified, "90 days")}
       GROUP BY m."type"

src/lib/insights/comprehensive-aggregator.ts

@@ -682,41 +682,54 @@ async function buildFromLiveAggregate(
           WHERE m."measured_at" >= NOW() - INTERVAL '60 days'
             AND m."measured_at" <  NOW() - INTERVAL '30 days'
         )::double precision                                           AS avg30_last_month,
+        -- A2-M2 — the regression windows anchor on the UTC-midnight day
+        -- boundary, NOT the wall-clock instant, so this cold-fallback path
+        -- returns the SAME boundary-day membership the warm rollup path
+        -- composes from (startOfUtcDay(now - N days) +
+        -- composeWindowedRegression). The expression
+        -- date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL 'N days'
+        -- is the SQL twin of startOfUtcDay; the trailing AT TIME ZONE 'UTC'
+        -- re-materialises the naive truncation as a UTC instant so the
+        -- measured_at (timestamptz) comparison is on the same wall the
+        -- rollup bucketStart sits on. Without this anchor an account that
+        -- crosses a coverage boundary (warm to cold) saw the boundary day
+        -- flip in/out of the slope window, giving a cache-dependent answer
+        -- to the same request.
         REGR_SLOPE(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '7 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '7 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS slope7,
         REGR_R2(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '7 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '7 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS r2_7,
         REGR_SLOPE(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '30 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '30 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS slope30,
         REGR_R2(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '30 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '30 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS r2_30,
         REGR_SLOPE(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '90 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '90 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS slope90,
         REGR_R2(
           m."value",
           EXTRACT(EPOCH FROM m."measured_at") / 86400.0
         ) FILTER (
-          WHERE m."measured_at" >= NOW() - INTERVAL '90 days'
+          WHERE m."measured_at" >= (date_trunc('day', NOW() AT TIME ZONE 'UTC') - INTERVAL '90 days') AT TIME ZONE 'UTC'
         )::double precision                                           AS r2_90
       FROM cm m
       JOIN window_stats ws ON ws."type" = m."type"