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.
daddy
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