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/api/endpoints/maintenance_schedules.py

@@ -76,6 +76,10 @@ async def create_schedule(
     db.add(schedule)
     await db.commit()
     await db.refresh(schedule)
+
+    from app.core.scheduler import register_schedule
+    register_schedule(schedule)
+
     return schedule
 
 
@@ -135,4 +139,11 @@ async def update_schedule(
 
     await db.commit()
     await db.refresh(schedule)
+
+    from app.core.scheduler import register_schedule, unregister_schedule
+    if schedule.is_active:
+        register_schedule(schedule)
+    else:
+        unregister_schedule(str(schedule.id))
+
     return schedule