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/main.py

@@ -6,11 +6,12 @@ from slowapi import _rate_limit_exceeded_handler
 from slowapi.errors import RateLimitExceeded
 
 from app.core.config import settings
-from app.core.database import init_db
+from app.core.database import init_db, async_session_maker
 from app.core.logging_config import setup_logging
 from app.core.middleware import RequestLoggingMiddleware, ErrorLoggingMiddleware
 from app.core.rate_limit import limiter
 from app.api.router import api_router
+from app.core.scheduler import scheduler, load_all_schedules
 
 # Initialize logging configuration
 setup_logging()
@@ -26,8 +27,16 @@ async def lifespan(app: FastAPI):
     logger.info(f"ALLOW_RAILWAY_ORIGINS: {settings.ALLOW_RAILWAY_ORIGINS}")
     # Note: In production, use Alembic migrations instead of init_db
     # await init_db()
+
+    # Start maintenance schedule runner
+    scheduler.start()
+    async with async_session_maker() as db:
+        await load_all_schedules(db)
+
     yield
+
     # Shutdown
+    scheduler.shutdown(wait=False)
     logger.info("Shutting down ResolutionFlow API server...")