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>

migrations/versions/2025_01_15_0400-c3d4e5f6g7h8_add_print_jobs.py

@@ -0,0 +1,53 @@
+"""add_print_jobs.
+
+Revision ID: c3d4e5f6g7h8
+Revises: b2c3d4e5f6g7
+Create Date: 2025-01-15 04:00:00.000000
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "c3d4e5f6g7h8"
+down_revision = "b2c3d4e5f6g7"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    """Create print_job table and add needs_weighing to spool."""
+    # Create print_job table
+    op.create_table(
+        "print_job",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("spool_id", sa.Integer(), nullable=False),
+        sa.Column("created", sa.DateTime(), nullable=False),
+        sa.Column("finished", sa.DateTime(), nullable=True),
+        sa.Column("filename", sa.String(length=256), nullable=False),
+        sa.Column("filament_used_g", sa.Float(), nullable=False),
+        sa.Column("filament_used_mm", sa.Float(), nullable=True),
+        sa.Column("status", sa.String(length=16), nullable=False, server_default="pending"),
+        sa.ForeignKeyConstraint(
+            ["spool_id"],
+            ["spool.id"],
+            ondelete="CASCADE",
+        ),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index(op.f("ix_print_job_id"), "print_job", ["id"], unique=False)
+    op.create_index(op.f("ix_print_job_spool_id"), "print_job", ["spool_id"], unique=False)
+
+    # Add needs_weighing column to spool table
+    op.add_column(
+        "spool",
+        sa.Column("needs_weighing", sa.Boolean(), nullable=True, server_default="0"),
+    )
+
+
+def downgrade() -> None:
+    """Drop print_job table and remove needs_weighing from spool."""
+    op.drop_column("spool", "needs_weighing")
+    op.drop_index(op.f("ix_print_job_spool_id"), table_name="print_job")
+    op.drop_index(op.f("ix_print_job_id"), table_name="print_job")
+    op.drop_table("print_job")