Add social graph models: friendships, follows, blocks

Models + Alembic migration (0007, head) only, per docs/planning/social.md
US-1 -- the friends/follow API router is a separate sibling task.

- friendships: requester_id/addressee_id (FK users, CASCADE), status
  (pending/accepted native enum), unique pair constraint, indexed FKs.
- follows: follower_id/followed_id (FK users, CASCADE), unique pair.
- blocks: blocker_id/blocked_id (FK users, CASCADE), unique pair.
- Relationships wired onto User for both directions of each table.
- tests/test_social_models.py exercises request/accept, the unique-pair
  constraint, follow, block, and DB-level CASCADE delete against real
  Postgres. Full backend suite (152 tests) still green.

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

backend/app/models/__init__.py

@@ -4,8 +4,10 @@ from app.models.world import World
 from app.models.asset import Asset
 from app.models.avatar_loadout import AvatarLoadout
 from app.models.world_content import WorldRoom, WorldChunk, WorldChunkRevision
+from app.models.social import Friendship, Follow, Block, FriendshipStatus
 
 __all__ = [
     "User", "Session", "World", "Asset", "AvatarLoadout",
     "WorldRoom", "WorldChunk", "WorldChunkRevision",
+    "Friendship", "Follow", "Block", "FriendshipStatus",
 ]