feat(db): Add favicon support and improve remote followers uniqueness constraint

- Add favicon_url column to nodes table for storing node instance favicons
- Replace actor_url unique constraint with composite unique index on (user_id, actor_url) in remote_followers table
- Update database schema and migration files to reflect new constraints
- Improve data integrity by ensuring remote followers are unique per user and actor combination

drizzle/meta/_journal.json

@@ -36,6 +36,13 @@
       "when": 1769377790354,
       "tag": "0004_luxuriant_lockheed",
       "breakpoints": true
+    },
+    {
+      "idx": 5,
+      "version": "7",
+      "when": 1769422771465,
+      "tag": "0005_yummy_millenium_guard",
+      "breakpoints": true
     }
   ]
 }