Add DM chat send/list endpoints: filter, rate limit, conversations (US-4)

Implements direct-message (DM) chat send and history endpoints:
- Migration 0011_add_dm_last_read.py (last_read_at on conversation_participants)
- Models updated in backend/app/models/chat.py
- Router in backend/app/routers/chat_dm.py:
  - POST /api/chat/dm/{user_id}/messages (send DM, filtered via TextFilter, rate-limited via Redis)
  - GET  /api/chat/dm/{user_id}/messages (paginated history, updates last_read_at)
  - GET  /api/chat/dm/conversations (conversations list, last message, unread count)
- Blocks enforced in both directions (cannot send to/from blocked users)
- Self-messaging prevented, peer exist check, auth required
- Integration tests in backend/tests/test_chat_dm.py

Independently re-verified: 14/14 new DM chat tests pass,
272/272 full backend regression suite passes with zero regressions.

backend/app/main.py

@@ -6,7 +6,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.config import settings
-from app.routers import auth, users, worlds, assets, avatar, world_content, internal, presence, groups, social_friends, profiles
+from app.routers import auth, users, worlds, assets, avatar, world_content, internal, presence, groups, social_friends, profiles, chat_dm
 
 
 from app.services.game_servers import supervisor
@@ -48,6 +48,7 @@ app.include_router(presence.router, prefix="/api/presence", tags=["presence"])
 app.include_router(groups.router, prefix="/api/groups", tags=["groups"])
 app.include_router(social_friends.router, prefix="/api/friends", tags=["friends"])
 app.include_router(profiles.router, prefix="/api/profile", tags=["profile"])
+app.include_router(chat_dm.router, prefix="/api/chat/dm", tags=["chat-dm"])
 
 
 @app.get("/api/health")