Add Print Job tracking for slicer integration

Backend for tracking print jobs from Elegoo/Orca Slicer:

- PrintJob model: tracks pending/completed/cancelled jobs
- API endpoints: create, list, complete, cancel jobs
- needs_weighing flag on Spool: for cancelled print recalibration
- Database migration for new table and column

Workflow:
1. Slicer post-processing script creates pending job
2. User completes job → auto-deducts filament
3. User cancels job → flags spool for manual weigh-in

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

spoolman/api/v1/router.py

@@ -15,7 +15,7 @@ from spoolman.database.database import backup_global_db
 from spoolman.exceptions import ItemNotFoundError
 from spoolman.ws import websocket_manager
 
-from . import backup as backup_module, export, externaldb, field, filament, models, other, setting, spool, vendor
+from . import backup as backup_module, export, externaldb, field, filament, models, other, print_job, setting, spool, vendor
 
 logger = logging.getLogger(__name__)
 
@@ -113,3 +113,4 @@ app.include_router(other.router)
 app.include_router(externaldb.router)
 app.include_router(export.router)
 app.include_router(backup_module.router)
+app.include_router(print_job.router)