Add chat data models + migration (conversations, messages, reports)

docs/planning/social.md US-4, task 6: conversations,
conversation_participants, messages (DM + world/room chat), and
message_reports, following app/models/social.py's conventions (native
enums, explicit index/constraint names, CASCADE FKs where appropriate).

Delete behavior, chosen per-relationship:
- conversations.user_a_id/user_b_id -> CASCADE: a DM is meaningless with
  one of its two participants gone, so it cascades away entirely
  (participants + messages with it).
- messages.sender_id -> SET NULL: world/room chat history and reported
  messages must survive the sender's account being deleted, matching the
  existing actor-reference convention (WorldChunk.updated_by_id,
  WorldChunkRevision.actor_id).
- messages.world_id/room_id -> CASCADE: chat scoped to a world/room has no
  meaning once that world/room is gone (mirrors WorldChunk).
- message_reports.message_id -> SET NULL (not CASCADE): a moderation
  report must outlive the message it's about, mirroring
  WorldChunkRevision's audit-trail-survives-deletion precedent.
- message_reports.reporter_id/moderator_id -> SET NULL: audit-actor
  references, same reasoning.

Models + migration only, no endpoints, no text filtering (sibling tasks).

Verified: migration upgrade/downgrade/upgrade round-trip against a real
Postgres with schema inspected via \d against spec; new model tests cover
DM creation, pair uniqueness + ordering check constraint, world/room
messages, report defaults, and cascade behavior for both
conversation-deletion and sender-deletion; full backend suite (160 tests)
passes with no regressions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2FESCaMXeJpC9gSctiEeD

backend/tests/conftest.py

@@ -45,6 +45,11 @@ TEST_TABLES = (
     # Social graph (app/models/social.py) -- all three FK users with
     # ON DELETE CASCADE, but list explicitly for clarity/consistency.
     "friendships", "follows", "blocks",
+    # Chat (app/models/chat.py). message_reports.message_id is ON DELETE SET
+    # NULL (not CASCADE, deliberately -- see chat.py), so it wouldn't be
+    # reached by cascading a truncate of messages/conversations; list it
+    # explicitly along with the rest for clarity/consistency.
+    "message_reports", "messages", "conversation_participants", "conversations",
     "sessions", "refresh_tokens", "users",
 )