Review of the slot-convergence change surfaced gaps that could mis-record
a dose. Fold the write logic into one shared `applyCanonicalSlotWrite`
used by both intake routes and close them:
- Concurrency: catch the unique-key race on insert and converge by
re-finding the slot row and updating it, instead of returning 500 (and
dropping the tap) or leaving a duplicate.
- No downgrade: a pending projection echo replayed onto a slot already
marked taken or skipped is a no-op — it never clears `takenAt`. A
recorded dose is immutable, so an offline re-sync cannot flip a taken
dose back to missed.
- Inventory: decrement a medication's remaining count only on a genuine
pending→taken transition, not on an idempotent re-post.
- Deterministic slot-row selection: prefer an existing actioned row, then
the pending row, ordered by createdAt then id.
- Snap instant: derive the canonical instant via the same `localHmAsUtc`
the projector and reminder worker mint with, so the value is identical
by construction across a DST transition rather than only test-pinned.
- Cleanup job: isolate each slot's collapse in its own try/catch and
guard the instant-normalisation against a tombstone collision, so one
slot can no longer dead-letter a user's whole dedup pass.
Marc-André Bombeck
710902a43c