feat: persistent chat message storage + Android pull-to-load history

Backend:
- Add ChatMessage model + Alembic migration 024
- Add on_message callback to AgentRuntime for persisting messages during SSE streaming
- Plumb session_id from ChatRequest to AgentContext in all 4 chat endpoints
- Add GET /agent-chat/{id}/sessions and /sessions/{sid}/messages with cursor pagination

Android:
- Add DTOs/ApiService/MessageDao for server-side chat history
- ChatRepository: fetchOlderMessages (API + Room cache), offline fallback
- ChatViewModel: loadMoreHistory with isLoadingMore/hasMoreMessages state
- ChatScreen: scroll-to-top detection + top loading indicator

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/database.py

@@ -76,4 +76,5 @@ def init_db():
     import app.models.workspace
     import app.models.scene_contract
     import app.models.team
+    import app.models.chat_message
     Base.metadata.create_all(bind=engine)