chat: durable Postgres event queue + cursor replay on reconnect (Phase 2+3)

Completes the turn/session decoupling so nothing is lost across disconnects.

Phase 2 (durability):
- New chat_session_events table (global monotonic id = cursor) + queries
  appendChatEvent / getChatEventsSince / pruneChatEventsOlderThan.
- Every durable outbound ServerMessage now goes through emitToSession: appended
  to the queue (even while the client is disconnected) and delivered live with
  its seq. Streaming deltas stay ephemeral (live-only, never persisted).

Phase 3 (resilient transport):
- New 'resume-cursor' client message → handleResumeCursor re-binds the socket to
  the (still-live) session (cancels idle-GC via attachWs) and replays every event
  since the client's cursor.
- useChatWebSocket already auto-reconnects; added an onOpen hook. useChat tracks
  the max seq and, on every (re)connect with an established session, sends
  resume-cursor — so a dropped connection self-heals with no manual navigate
  away/back, and background task notifications that landed while offline replay.

Verified end-to-end: disconnect after a turn's result but before a background
task finishes, reconnect with the cursor → the missed task:notification is
replayed from Postgres, no duplicates.

Note: the DB is managed via drizzle push/direct DDL (no __drizzle_migrations
table), so 0001 was applied directly; the generated migration is committed for
the record.

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

src/databases/officer_db/migrations/meta/_journal.json

@@ -8,6 +8,13 @@
       "when": 1785018112373,
       "tag": "0000_eager_sir_ram",
       "breakpoints": true
+    },
+    {
+      "idx": 1,
+      "version": "7",
+      "when": 1785110579165,
+      "tag": "0001_omniscient_photon",
+      "breakpoints": true
     }
   ]
 }