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/models/chat.py

@@ -121,6 +121,10 @@ class ConversationParticipant(Base):
         Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True
     )
     created_at = Column(DateTime(timezone=True), server_default=func.now(), nullable=False)
+    # DM unread tracking (app/routers/chat_dm.py, migration 0010): bumped to
+    # "now" whenever this participant fetches the conversation's history.
+    # Null means "never read" -- everything in the conversation is unread.
+    last_read_at = Column(DateTime(timezone=True), nullable=True)
 
     conversation = relationship("Conversation", back_populates="participants")
     user = relationship("User")