New `syncUserSleep` routine pulls per-segment sleep data from
POST /v2/sleep?action=get and writes one Measurement row per stage
segment, tagged with the appropriate SleepStage enum:
state 0 (awake) → AWAKE
state 1 (light sleep) → CORE (HealthKit-aligned NREM 1+2)
state 2 (deep sleep) → DEEP
state 3 (REM) → REM
state 4 (synthetic) → ignored
The legacy ASLEEP and IN_BED enum slots stay reserved for HealthKit
ingest (pre-iOS-16 `asleepUnspecified` and iOS-16+ `inBed`).
ScanWatch nights with no REM segment simply omit the row rather than
synthesising an all-zeros entry — Withings drops the field, the writer
never sees it.
Idempotency rides on Migration 0055's five-column composite — a re-sync
of the same night UPDATEs every segment row in place rather than
inserting duplicates, even when Withings re-aggregates the night with
adjusted segment boundaries.
Sleep durations stored as canonical minutes (seconds ÷ 60, rounded) so
re-syncs don't flip values on floating-point noise.