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/core/scheduler.py

@@ -0,0 +1,139 @@
+"""APScheduler integration for maintenance flow auto-session creation."""
+import logging
+import uuid
+from datetime import datetime, timezone
+
+from apscheduler.schedulers.asyncio import AsyncIOScheduler
+from apscheduler.triggers.cron import CronTrigger
+import pytz
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+logger = logging.getLogger(__name__)
+
+scheduler = AsyncIOScheduler()
+
+
+async def _fire_maintenance_schedule(schedule_id: str) -> None:
+    """Create batch sessions for a scheduled maintenance run."""
+    # Import all models first to ensure SQLAlchemy mapper relationships resolve
+    import app.models  # noqa: F401
+    from app.core.database import async_session_maker
+    from app.models.maintenance_schedule import MaintenanceSchedule
+    from app.models.session import Session
+    from app.models.target_list import TargetList
+    from app.models.tree import Tree
+
+    async with async_session_maker() as db:
+        try:
+            result = await db.execute(
+                select(MaintenanceSchedule).where(
+                    MaintenanceSchedule.id == uuid.UUID(schedule_id),
+                    MaintenanceSchedule.is_active == True,
+                )
+            )
+            schedule = result.scalar_one_or_none()
+            if not schedule:
+                logger.warning(f"Schedule {schedule_id} not found or inactive")
+                return
+
+            tree_result = await db.execute(
+                select(Tree).where(Tree.id == schedule.tree_id)
+            )
+            tree = tree_result.scalar_one_or_none()
+            if not tree:
+                logger.error(f"Tree {schedule.tree_id} not found for schedule {schedule_id}")
+                return
+
+            # Resolve targets
+            targets: list[dict] = []
+            if schedule.target_list_id:
+                list_result = await db.execute(
+                    select(TargetList).where(TargetList.id == schedule.target_list_id)
+                )
+                target_list = list_result.scalar_one_or_none()
+                if target_list:
+                    targets = list(target_list.targets)
+
+            if not targets:
+                targets = [{"label": "Unassigned"}]
+
+            batch_id = uuid.uuid4()
+            tree_snapshot = tree.tree_structure
+
+            sessions_to_add = []
+            for target in targets:
+                session = Session(
+                    tree_id=tree.id,
+                    user_id=schedule.created_by,
+                    tree_snapshot=tree_snapshot,
+                    path_taken=[],
+                    decisions=[],
+                    custom_steps=[],
+                    session_variables={},
+                    batch_id=batch_id,
+                    target_label=target.get("label", ""),
+                )
+                sessions_to_add.append(session)
+
+            for s in sessions_to_add:
+                db.add(s)
+
+            # Update schedule tracking
+            schedule.last_run_at = datetime.now(timezone.utc)
+            from croniter import croniter
+            tz = pytz.timezone(schedule.timezone)
+            now = datetime.now(tz)
+            cron = croniter(schedule.cron_expression, now)
+            schedule.next_run_at = cron.get_next(datetime).astimezone(timezone.utc)
+
+            await db.commit()
+            logger.info(
+                f"Schedule {schedule_id} fired: created {len(sessions_to_add)} sessions "
+                f"(batch {batch_id}) for tree '{tree.name}'"
+            )
+        except Exception:
+            logger.exception(f"Error firing maintenance schedule {schedule_id}")
+            await db.rollback()
+
+
+async def load_all_schedules(db: AsyncSession) -> None:
+    """Load all active schedules into APScheduler on startup."""
+    # Import all models to ensure SQLAlchemy mapper relationships resolve
+    # before any ORM queries are executed.
+    import app.models  # noqa: F401
+    from app.models.maintenance_schedule import MaintenanceSchedule
+    result = await db.execute(
+        select(MaintenanceSchedule).where(MaintenanceSchedule.is_active == True)
+    )
+    schedules = result.scalars().all()
+    for schedule in schedules:
+        register_schedule(schedule)
+    logger.info(f"Loaded {len(schedules)} active maintenance schedule(s)")
+
+
+def register_schedule(schedule) -> None:
+    """Register or update a schedule in APScheduler."""
+    job_id = f"maintenance_{schedule.id}"
+    try:
+        tz = pytz.timezone(schedule.timezone)
+        trigger = CronTrigger.from_crontab(schedule.cron_expression, timezone=tz)
+        scheduler.add_job(
+            _fire_maintenance_schedule,
+            trigger=trigger,
+            id=job_id,
+            args=[str(schedule.id)],
+            replace_existing=True,
+            misfire_grace_time=3600,
+        )
+        logger.info(f"Registered schedule {schedule.id} ({schedule.cron_expression})")
+    except Exception:
+        logger.exception(f"Failed to register schedule {schedule.id}")
+
+
+def unregister_schedule(schedule_id: str) -> None:
+    """Remove a schedule from APScheduler."""
+    job_id = f"maintenance_{schedule_id}"
+    if scheduler.get_job(job_id):
+        scheduler.remove_job(job_id)
+        logger.info(f"Unregistered schedule {schedule_id}")