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/models.py

@@ -331,6 +331,10 @@ class Spool(BaseModel):
         examples=[""],
     )
     archived: bool = Field(description="Whether this spool is archived and should not be used anymore.")
+    needs_weighing: bool = Field(
+        default=False,
+        description="Whether this spool needs manual weigh-in (e.g., after a cancelled print).",
+    )
     extra: dict[str, str] = Field(
         description=(
             "Extra fields for this spool. All values are JSON-encoded data. "
@@ -384,6 +388,7 @@ class Spool(BaseModel):
             lot_nr=item.lot_nr,
             comment=item.comment,
             archived=item.archived if item.archived is not None else False,
+            needs_weighing=item.needs_weighing if item.needs_weighing is not None else False,
             extra={field.key: field.value for field in item.extra},
         )