fix(sleep): reconstruct an ordered WHOOP hypnogram timeline

WHOOP's v2 API returns only per-stage sleep duration totals — no onset
timestamps, no hypnogram endpoint. The mapper stamped all five stage
totals on the one sleep-END instant, so the hypnogram reconstructed
every stage as a span touching the night's right edge: bars stacked
right-aligned with no clock times whenever WHOOP was the sole source.

Lay the asleep stages back-to-back from sleep onset in a fixed
physiological order (AWAKE lead, then CORE → DEEP → REM), emitting one
timed row per segment with measuredAt at that segment's end and a
distinct indexed externalId. IN_BED stays a single envelope row over the
whole window. The existing reader and renderer are untouched — they now
see distinct per-segment instants and place each block at its own time.

The order is synthesised, not measured, so every reconstructed row is
flagged and the night DTO carries a per-session `reconstructed` boolean
(GET /api/sleep/night, regenerated OpenAPI). The client renders the
hypnogram but labels it an approximate layout and never recomputes;
real-series sources (Apple Health, Withings, Fitbit) stay false.

src/app/api/sleep/night/route.ts

@@ -12,6 +12,11 @@
  * priority ladder — so two sources' timelines never overlay. A view over the
  * existing per-stage `SLEEP_DURATION` rows; no schema, no new type.
  *
+ * `reconstructed` (per session) is true when the winning source has no
+ * per-stage onset timestamps and the server synthesised a contiguous timeline
+ * (WHOOP) — the client renders the hypnogram but labels it an approximate
+ * layout. A real-series source (Apple Health / Withings / Fitbit) is false.
+ *
  * `date` omitted → the most recent night in the trailing-year window.
  *
  * Auth: cookie session OR Bearer token (`requireAuth`). Soft-delete-filtered.
@@ -64,6 +69,10 @@ function serializeSession(s: SleepSession) {
     inBedMinutes: s.inBedMinutes === null ? null : Math.round(s.inBedMinutes),
     awakeMinutes: s.awakeMinutes === null ? null : Math.round(s.awakeMinutes),
     awakenings: s.awakenings,
+    // iOS #18 — `reconstructed` is true when the source has no per-stage onset
+    // timestamps and the server synthesised a contiguous timeline (WHOOP). iOS
+    // labels the hypnogram as an approximate layout; it never recomputes.
+    reconstructed: s.reconstructed,
     // iOS #18 — round the per-stage map too; the underlying totals sum
     // second-precision segments and otherwise serialise as e.g. 88.4999.
     stages: Object.fromEntries(