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/database/models.py

@@ -80,6 +80,10 @@ class Spool(Base):
     lot_nr: Mapped[Optional[str]] = mapped_column(String(64))
     comment: Mapped[Optional[str]] = mapped_column(String(1024))
     archived: Mapped[Optional[bool]] = mapped_column()
+    needs_weighing: Mapped[Optional[bool]] = mapped_column(
+        default=False,
+        comment="Flag indicating spool needs manual weigh-in (e.g., after cancelled print).",
+    )
     extra: Mapped[list["SpoolField"]] = relationship(
         back_populates="spool",
         cascade="save-update, merge, delete, delete-orphan",
@@ -106,6 +110,26 @@ class SpoolAdjustment(Base):
     comment: Mapped[Optional[str]] = mapped_column(String(256), comment="Optional user comment.")
 
 
+class PrintJob(Base):
+    """Track pending/completed print jobs from slicer."""
+
+    __tablename__ = "print_job"
+
+    id: Mapped[int] = mapped_column(primary_key=True, index=True)
+    spool_id: Mapped[int] = mapped_column(ForeignKey("spool.id", ondelete="CASCADE"), index=True)
+    spool: Mapped["Spool"] = relationship()
+    created: Mapped[datetime] = mapped_column(comment="When the job was created (slice time).")
+    finished: Mapped[Optional[datetime]] = mapped_column(comment="When the job was completed/cancelled.")
+    filename: Mapped[str] = mapped_column(String(256), comment="G-code filename.")
+    filament_used_g: Mapped[float] = mapped_column(comment="Estimated filament usage in grams.")
+    filament_used_mm: Mapped[Optional[float]] = mapped_column(comment="Estimated filament usage in mm.")
+    status: Mapped[str] = mapped_column(
+        String(16),
+        default="pending",
+        comment="Status: 'pending', 'completed', 'cancelled'.",
+    )
+
+
 class Setting(Base):
     __tablename__ = "setting"