feat(environment): resolve past days conservatively, never guessing from home

Rework the per-day location resolution so it can no longer fabricate weather for
days before the home was set. Precedence, first hit wins:

  1. an explicit dated location period (a trip, or a corrected stretch of
     history) covering the day;
  2. DEVICE — a reserved slot for a future client-supplied coarse per-day
     location (ranks between the period and home; not produced server-side yet);
  3. the home location, but only for days on/after its effective-from date;
  4. otherwise the day is skipped.

The home route stamps `homeSince` whenever the home is set/updated and returns
it. The overview exposes it. Backfill bounds are now optional and default to the
conservative `[homeSince .. today]` span instead of a fixed reach onto the
current home; days before `homeSince` still resolve to a skip in the worker, so
even a wider explicit start cannot back-fill weather for the pre-home past. To
reach further back a user adds an explicit location period for that range.

Add unit coverage for the precedence (including the effective-from boundary and
the skip-not-guess rule) and the default backfill range.

src/app/api/environment/backfill/route.ts

@@ -21,7 +21,11 @@ import { requireModuleEnabled } from "@/lib/modules/gate";
 import { checkAnalyticsReadRateLimit } from "@/lib/rate-limit";
 import { prisma } from "@/lib/db";
 import { environmentBackfillSchema } from "@/lib/validations/environment";
-import { ENVIRONMENT_MAX_BACKFILL_DAYS } from "@/lib/environment/service";
+import {
+  ENVIRONMENT_MAX_BACKFILL_DAYS,
+  defaultBackfillRange,
+  utcDayKey,
+} from "@/lib/environment/service";
 import { enqueueEnvironmentFetch } from "@/lib/jobs/environment-fetch";
 
 const MS_PER_DAY = 24 * 60 * 60 * 1000;
@@ -49,7 +53,33 @@ export const POST = apiHandler(async (request: NextRequest) => {
     });
   }
 
-  const { startDate, endDate } = parsed.data;
+  const profile = await prisma.user.findUnique({
+    where: { id: user.id },
+    select: { homeLat: true, homeSince: true },
+  });
+  if (profile?.homeLat == null) {
+    return apiError("Set a home location before backfilling.", 409, {
+      errorCode: "environment.no_home",
+    });
+  }
+
+  // Resolve the effective span. An omitted bound defaults to the conservative
+  // [homeSince .. today] range — never a fixed reach into the past onto the
+  // current home. Days before homeSince still resolve to SKIP in the worker, so
+  // even an explicit wider start cannot fabricate weather for the pre-home past.
+  const today = utcDayKey(new Date());
+  const fallback = defaultBackfillRange(profile.homeSince) ?? {
+    startDate: today,
+    endDate: today,
+  };
+  const startDate = parsed.data.startDate ?? fallback.startDate;
+  const endDate = parsed.data.endDate ?? fallback.endDate;
+  if (startDate > endDate) {
+    return apiError("startDate must be on or before endDate", 422, {
+      errorCode: "environment.invalid",
+    });
+  }
+
   const [sy, sm, sd] = startDate.split("-").map(Number);
   const [ey, em, ed] = endDate.split("-").map(Number);
   const spanDays =
@@ -64,16 +94,6 @@ export const POST = apiHandler(async (request: NextRequest) => {
     );
   }
 
-  const profile = await prisma.user.findUnique({
-    where: { id: user.id },
-    select: { homeLat: true },
-  });
-  if (profile?.homeLat == null) {
-    return apiError("Set a home location before backfilling.", 409, {
-      errorCode: "environment.no_home",
-    });
-  }
-
   const enqueued = await enqueueEnvironmentFetch({
     userId: user.id,
     startDate,

src/app/api/environment/home/route.ts

@@ -41,11 +41,16 @@ export const PUT = apiHandler(async (request: NextRequest) => {
   }
 
   const { lat, lon, label, timezone } = parsed.data;
+  // Stamp the effective-from instant: from now on this home resolves days; days
+  // before it stay un-attributed (filled via explicit location periods). Every
+  // set/update re-stamps — the home is "effective from the moment it is set".
+  const homeSince = new Date();
   const home = {
     homeLat: roundCoarse(lat),
     homeLon: roundCoarse(lon),
     homeLabel: label,
     homeTimezone: timezone,
+    homeSince,
   };
 
   await prisma.user.update({ where: { id: user.id }, data: home });
@@ -62,6 +67,7 @@ export const PUT = apiHandler(async (request: NextRequest) => {
       lon: home.homeLon,
       label: home.homeLabel,
       timezone: home.homeTimezone,
+      since: homeSince.toISOString(),
     },
   });
 });

src/app/api/environment/route.ts

@@ -29,6 +29,7 @@ export const GET = apiHandler(async () => {
         homeLon: true,
         homeLabel: true,
         homeTimezone: true,
+        homeSince: true,
         timezone: true,
       },
     }),
@@ -59,6 +60,9 @@ export const GET = apiHandler(async () => {
           lon: profile.homeLon,
           label: profile.homeLabel,
           timezone: profile.homeTimezone ?? profile.timezone,
+          // Effective-from instant: the settings surface uses it to label the
+          // home and to prefill the backfill start (conservative default range).
+          since: profile.homeSince?.toISOString() ?? null,
         }
       : null;