perf(coach): bound buildCoachSnapshot to last 90 days

Every POST /api/insights/chat called extractFeatures(userId, false),
which ran prisma.measurement.findMany({ where: { userId } }) with no
date filter — fetching every measurement the user had ever recorded.
For power users with multi-year Withings imports that's tens of
thousands of rows on every Coach turn.

Add an optional sinceDays bound to extractFeatures (default unchanged
= full history; analytics dashboard + insights/generate keep their
existing behaviour) and have the Coach snapshot pass sinceDays: 90.
The Coach already only narrates the last7 / last30 / last90 windows,
so the 90-day floor preserves every metric the snapshot surfaces.

Fixes Senior-HIGH-3.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/ai/coach/snapshot.ts

@@ -25,11 +25,21 @@ export interface CoachSnapshotResult {
  * `includeRaw=false` because the Coach replies are conversational and
  * the user is asking the model — they should never depend on raw
  * measurement timestamps that the privacy mode controls.
+ *
+ * Bounded to the last 90 days. Coach narrates the same windows the
+ * Daily Briefing renders (last7 / last30 / last90); fetching every
+ * historical measurement on each turn was unbounded I/O for power
+ * users with multi-year Withings imports. The 90-day floor is the
+ * widest window any Coach metric currently consumes.
  */
+const COACH_SNAPSHOT_WINDOW_DAYS = 90;
+
 export async function buildCoachSnapshot(
   userId: string,
 ): Promise<CoachSnapshotResult> {
-  const features = await extractFeatures(userId, false);
+  const features = await extractFeatures(userId, false, {
+    sinceDays: COACH_SNAPSHOT_WINDOW_DAYS,
+  });
 
   // Trim down to the metrics the Coach narrates. extractFeatures
   // returns more (sleep, steps, etc.) — the Coach surface keeps the

src/lib/insights/features.ts

@@ -256,6 +256,7 @@ function computeHistoricalComparison(
 export async function extractFeatures(
   userId: string,
   includeRaw: boolean,
+  options: { sinceDays?: number } = {},
 ): Promise<AggregatedFeatures | RawFeatures> {
   const user = await prisma.user.findUnique({
     where: { id: userId },
@@ -268,9 +269,21 @@ export async function extractFeatures(
 
   const now = Date.now();
 
-  // Fetch ALL measurements (not just 30 days) for full temporal context
+  // Fetch measurements. Default = ALL (full temporal context for the
+  // dashboard / insights generator). Callers that only consume the
+  // ≤90-day windows (Coach snapshot per turn) pass `sinceDays: 90` so
+  // the per-turn I/O stays bounded — `findMany({ where: { userId } })`
+  // is unbounded by user-history size and gets paid once per Coach turn
+  // for power users with multi-year Withings imports.
+  const sinceDays = options.sinceDays;
+  const sinceCutoff =
+    typeof sinceDays === "number" && sinceDays > 0
+      ? new Date(now - sinceDays * 24 * 60 * 60 * 1000)
+      : null;
   const measurements = await prisma.measurement.findMany({
-    where: { userId },
+    where: sinceCutoff
+      ? { userId, measuredAt: { gte: sinceCutoff } }
+      : { userId },
     orderBy: { measuredAt: "asc" },
   });