chore(release): v1.28.39 — sync data-loss hardening

Four verified HIGH data-integrity fixes, each in-place per-provider (no
shared helper), from the campaign's data-integrity audit + adversarial
verification:

F1 — drain-per-sample-cumulative buildScanWhere was the only one of four
sibling drains missing deletedAt: null; the nightly cumulative fold
re-summed soft-deleted Apple samples and hard-deleted the legacy-steps
audit tombstones. Added the filter (mirrors consolidate-daily-mean,
dense-intraday-retention, consolidate-legacy-steps).

F2 — the additive late-fold merge had no single-flight guard, so two
overlapping runs (deploy overlap, or the admin endpoint racing the cron)
produced a permanent 2x daily total. Added a pg_advisory_xact_lock per
(user, externalId) plus an in-transaction re-read of the live
contributors so the serialised-second fold contributes zero (a bare lock
alone would still re-add its stale scanned sum). Mirrors the documents
quota advisory-lock pattern.

F3 — Withings sleep externalId keyed on segment START while measuredAt +
the natural key use END; a re-scored segment missed the externalId probe,
the sweep tombstoned the old row, and the re-keyed insert collided on the
natural key and wedged. Added the natural-key rescue (re-key in place),
mirroring the same-file measure path.

F4 — Withings measure sync stamped lastSyncedAt + recordSyncSuccess
unconditionally after a warn-only per-row loop; a transient write failure
stranded the reading behind the 10-min overlap forever. Now holds the
watermark and records failure on a hard row error (mirrors
google-health/fitbit), while a benign P2002 still advances.

Regression test per finding (reverting the three source files fails
exactly four tests). Integration suite green against real Postgres.

docs/api/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.28.38
+  version: 1.28.39
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.