Persist chat conversations to database

- Add harness_chat_conversations table with messages stored as JSONB
- Add chat-store.ts with CRUD operations
- Add /api/conversations routes (GET list, POST create, PATCH update,
  DELETE remove)
- Update chat-tab.tsx to load conversations on mount, persist metadata
  changes immediately, and save messages after streaming completes
- Add loading state while conversations are being fetched

packages/db/drizzle/meta/_journal.json

@@ -22,6 +22,13 @@
       "when": 1774178806032,
       "tag": "0002_cold_annihilus",
       "breakpoints": true
+    },
+    {
+      "idx": 3,
+      "version": "7",
+      "when": 1774179559510,
+      "tag": "0003_damp_glorian",
+      "breakpoints": true
     }
   ]
 }