feat(environment): add home effective-from and reserve a DEVICE location source

Add `User.homeSince` (nullable effective-from instant) and a `DEVICE` value to
the location-source enum (migration 0219). `homeSince` records when the home
location became effective so day resolution can attribute a past day to the
home only on/after that instant, rather than to the current home for every past
day. Existing accounts with a home get `homeSince` stamped to the migration
instant; their deep past is filled by explicit location periods instead.

`DEVICE` is reserved for a future client-supplied coarse per-day location: the
enum value and the wire contract accept it now so the resolver can rank it
later without rework. Nothing writes it server-side yet.

prisma/migrations/0219_v125_env_home_since/migration.sql

@@ -0,0 +1,36 @@
+-- v1.25 (W-ENV) — conservative past-location handling for the environmental
+-- context module.
+--
+-- Adds `users.home_since` (effective-from instant for the home location) and a
+-- `DEVICE` value to the location-source enum. Day resolution attributes a past
+-- day to the home location ONLY when the day is on/after `home_since`; earlier
+-- days are skipped rather than fabricated from the current home. The deep past
+-- is filled by explicit dated location periods instead.
+--
+-- `DEVICE` is the reserved forward path for a future client-supplied coarse
+-- per-day location (iOS v2). The value exists so the resolver and the wire
+-- contract already accept it; nothing writes it server-side yet.
+--
+-- Backfill of existing accounts: any row that already has a home location gets
+-- `home_since` stamped to the migration instant. This is conservative — the
+-- exact day a pre-upgrade account moved into its home is unknown, so history
+-- before the upgrade is left to explicit location periods rather than guessed.
+--
+-- Additive; no existing weather row touched. Idempotent guards so reruns are
+-- safe on prod.
+--
+-- Reversibility (down):
+--   ALTER TABLE "users" DROP COLUMN IF EXISTS "home_since";
+--   -- (the `DEVICE` enum value cannot be dropped in place; it is inert until a
+--   --  future client writes it, so a roll-back leaves it harmless.)
+
+-- AlterTable: effective-from instant for the home location (nullable).
+ALTER TABLE "users" ADD COLUMN IF NOT EXISTS "home_since" TIMESTAMP(3);
+
+-- Backfill: stamp existing home rows so they resolve from the upgrade forward.
+UPDATE "users"
+  SET "home_since" = CURRENT_TIMESTAMP
+  WHERE "home_lat" IS NOT NULL AND "home_since" IS NULL;
+
+-- AlterEnum: reserve the DEVICE source for the iOS v2 forward path.
+ALTER TYPE "environment_location_source" ADD VALUE IF NOT EXISTS 'DEVICE';

prisma/schema.prisma

@@ -63,6 +63,12 @@ model User {
   homeLon      Float?  @map("home_lon")
   homeLabel    String? @map("home_label")
   homeTimezone String? @map("home_timezone")
+  // v1.25 — effective-from instant for the home location. Stamped the moment a
+  // home is set/updated. Day resolution attributes a past day to home ONLY when
+  // the day is on/after this instant; earlier days are skipped (never fabricated
+  // from the current home). To fill the deep past the user adds an explicit
+  // dated location period instead. Backfill defaults its start to this instant.
+  homeSince    DateTime? @map("home_since")
 
   // Codex OAuth (ChatGPT subscription)
   codexAccessTokenEncrypted  String?   @map("codex_access_token_encrypted")
@@ -811,11 +817,16 @@ model User {
 // ─── v1.25 (W-ENV) Environmental-context module ──────────────────────────────
 
 /// How the location for an environment observation was resolved. HOME = the
-/// user's configured home location; TRAVEL = a manual date-range override.
-/// (iOS coarse device location is a v2 follow-up and not represented here.)
+/// user's configured home location; TRAVEL = an explicit dated location period
+/// (a past trip OR a corrected stretch of history) that overrides home for the
+/// days it covers; DEVICE = a future client-supplied coarse per-day location.
+/// DEVICE is the priority forward path for the iOS v2 client: the value exists
+/// so the resolver and the wire contract already accept it, but nothing writes
+/// it server-side yet.
 enum EnvironmentLocationSource {
   HOME
   TRAVEL
+  DEVICE
 }
 
 /// A per-day environmental observation (daily weather / daylight) for the
@@ -876,10 +887,13 @@ model EnvironmentContext {
   @@map("environment_contexts")
 }
 
-/// A manual travel override: for any day in `[startDate, endDate]` the weather
-/// fetch resolves against this location instead of the home location. Declared,
-/// not inferred — a stated trip dominates the home fallback so a winter week in
-/// a sunny locale is not mis-attributed to the home city. Coarse location only.
+/// An explicit dated LOCATION PERIOD: for any day in `[startDate, endDate]` the
+/// weather fetch resolves against this location instead of the home location.
+/// Declared, not inferred — it dominates the home fallback so a stretch spent
+/// elsewhere is not mis-attributed to the home city. Two uses share this one
+/// model: a discrete trip away from home, AND the mechanism to correct history
+/// (e.g. a whole-history home city for the years before the current home became
+/// effective). Coarse location only. (Stored under `source = TRAVEL`.)
 model EnvironmentTravelLocation {
   id     String @id @default(cuid())
   userId String @map("user_id")