Add groups + group_members models and migration

docs/planning/social.md US-5 (task 13). Models + Alembic migration only,
per the sibling-dependency boundary with the Groups API task.

- groups: id, name, description, owner_id (FK users, ON DELETE SET NULL --
  a group is a durable multi-member community, so it and its full roster
  survive the owner deleting their account; the API task handles surfacing
  an ownerless group for reassignment).
- group_members: group_id/user_id (both FK, ON DELETE CASCADE -- pure
  join-table rows with no meaning without either side), role
  (owner/admin/member native enum, default member), joined_at, unique
  constraint on (group_id, user_id).
- User gets owned_groups / group_memberships relationships.
- tests/conftest.py TEST_TABLES updated for truncation between tests.
- New tests/test_group_models.py: role assignment, unique-membership
  rejection, group-delete cascade, member-delete cascade, and owner-delete
  SET NULL survival -- all verified against real Postgres.

Verified: upgrade/downgrade/upgrade round-trip, \d-inspected schema matches
spec (FKs, cascade behavior, unique constraint, indexes), full backend
suite 166 passed / 0 regressions.

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

backend/app/models/__init__.py

@@ -8,10 +8,12 @@ from app.models.social import Friendship, Follow, Block, FriendshipStatus
 from app.models.chat import (
     Conversation, ConversationParticipant, Message, MessageReport, ReportStatus,
 )
+from app.models.groups import Group, GroupMember, GroupRole
 
 __all__ = [
     "User", "Session", "World", "Asset", "AvatarLoadout",
     "WorldRoom", "WorldChunk", "WorldChunkRevision",
     "Friendship", "Follow", "Block", "FriendshipStatus",
     "Conversation", "ConversationParticipant", "Message", "MessageReport", "ReportStatus",
+    "Group", "GroupMember", "GroupRole",
 ]