feat: APScheduler integration for maintenance flow auto-session creation

- Add backend/app/core/scheduler.py with AsyncIOScheduler, CronTrigger-based
  job registration, and _fire_maintenance_schedule to create batch sessions
- Wire scheduler.start()/load_all_schedules()/shutdown() into main.py lifespan
- Call register_schedule() in create_schedule endpoint after commit
- Call register_schedule()/unregister_schedule() in update_schedule based on is_active
- Add TreeShare to models/__init__.py so all SQLAlchemy mapper relationships
  resolve before ORM queries in the scheduler context

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -5,6 +5,7 @@ from .subscription import Subscription
 from .plan_limits import PlanLimits
 from .account_invite import AccountInvite
 from .tree import Tree
+from .tree_share import TreeShare
 from .session import Session
 from .attachment import Attachment
 from .invite_code import InviteCode
@@ -33,6 +34,7 @@ __all__ = [
     "PlanLimits",
     "AccountInvite",
     "Tree",
+    "TreeShare",
     "Session",
     "Attachment",
     "InviteCode",