Record sent chats in messages repo and blur gallery by default

Threads MessagesRepository through ChatService so first outgoing chat
messages are persisted for stats, and extends the repo + DB client to
support chat-room-keyed lookups. Adds tests covering the new behavior.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/database.test.ts

@@ -130,7 +130,8 @@ describe('Database Client', () => {
       expect(columnMap.get('subject')?.notnull).toBe(1);
       expect(columnMap.get('message')?.notnull).toBe(1);
       expect(columnMap.get('sent_at')?.notnull).toBe(1);
-      expect(columnMap.get('post_id')?.notnull).toBe(1);
+      expect(columnMap.get('post_id')?.notnull).toBe(0); // nullable — chat rows have no post
+      expect(columnMap.get('chat_room_id')).toBeDefined();
     });
 
     it('successful_messages has correct schema', () => {