feat(fleet): native Mosaic backlog on @mosaicstack/db (atomic claim + TTL)

Add Mosaic's own backlog-of-record on the existing Postgres storage layer,
replacing the former Hermes adapter (no Hermes dependency) and the earlier
sqlite idea (no sqlite, no new client).

- packages/db: `backlog` table (drizzle) + enum + migration 0011; BacklogService
  taking a Db handle so it runs on both createDb (server PG) and createPgliteDb
  (embedded). Atomic claim via SELECT ... FOR UPDATE SKIP LOCKED inside one tx,
  TTL claims + reclaim of expired, deps DAG gate, idempotency-key dedup, stats.
- packages/mosaic: `mosaic fleet backlog <sub> --json` (create/list/claim/
  reclaim/link/stats/block/complete). Embedded PGlite default at
  <mosaicHome>/fleet/backlog; full Postgres via DATABASE_URL — same code.
  Migrations run on first use.
- install.sh: preserve fleet/backlog across `mosaic update`.
- docs/fleet/backlog-conventions.md: schema, phase convention, atomic-claim +
  TTL semantics, PGlite-default/Postgres-by-config, Mosaic-native (no Hermes).
- Tests (in-memory PGlite, real PG semantics): create/list, exactly-one-winner
  concurrency (and N-card no-double-claim), deps gate, reclaim of expired, stats.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/db/drizzle/meta/_journal.json

@@ -78,6 +78,13 @@
       "when": 1745366400000,
       "tag": "0010_federation_enrollment_tokens",
       "breakpoints": true
+    },
+    {
+      "idx": 11,
+      "version": "7",
+      "when": 1782310438919,
+      "tag": "0011_bitter_gateway",
+      "breakpoints": true
     }
   ]
-}
+}