Phase 2: private rooms, room roles, and invites (backend only)

Adds room_invites table + migration, owner/admin/member role enforcement
(require_room_role), and endpoints for private room creation, room
management (update/delete/leave/transfer-ownership/change-role/remove-member),
and the invite lifecycle (create/list/accept/decline/revoke). Registration
stays invite-only via the CLI from Phase 1 — this is a separate, room-level
invite system for adding existing users to private rooms.

Frontend is untouched: the UI redesign is happening separately, so this phase
is backend + tests only (35 passing). Verified no regressions in the Phase 1
open-room/WebSocket flow via manual smoke test.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -1,7 +1,17 @@
 from app.models.base import Base
+from app.models.invite import InviteStatus, RoomInvite
 from app.models.membership import RoomMembership, RoomRole
 from app.models.message import Message
 from app.models.room import Room
 from app.models.user import User
 
-__all__ = ["Base", "User", "Room", "RoomMembership", "RoomRole", "Message"]
+__all__ = [
+    "Base",
+    "User",
+    "Room",
+    "RoomMembership",
+    "RoomRole",
+    "Message",
+    "RoomInvite",
+    "InviteStatus",
+]