Mirrors flutter_client/lib/cache/db.dart's CachedMutations — the
offline-write queue that MutationQueue.enqueue() inserts into when a
server-write fails with IOException and MutationReplayer.drain() pops
from when connectivity returns (Phase 12.2).
`kind` is a string registered in `MutationKind` (Phase 12.2) so the
replayer can map to the right handler. `payload` is JSON-serialized
args. Unknown kinds get dropped at drain time rather than wedging.
DAO surface tailored to the replayer:
- observePendingCount: Flow<Int> for the offline-indicator badge
- getAll: FIFO list for drain (id ASC = oldest first)
- insert: returns the autoGenerate'd id
- recordAttempt(id, instant): atomic increment + lastAttemptAt set
- delete(id) / clear
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>