Make printer optional on job creation — assigned when job starts

Migration 0005 makes print_jobs.printer_id nullable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

alembic/versions/0005_nullable_printer_id.py

@@ -0,0 +1,28 @@
+"""make print_jobs.printer_id nullable — printer is assigned at job start
+
+Revision ID: 0005
+Revises: 0004
+Create Date: 2026-06-18
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0005"
+down_revision: Union[str, None] = "0004"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.alter_column("print_jobs", "printer_id",
+                    existing_type=sa.Integer(),
+                    nullable=True)
+
+
+def downgrade() -> None:
+    op.alter_column("print_jobs", "printer_id",
+                    existing_type=sa.Integer(),
+                    nullable=False)