feat: add partial shipment support (Phase 3)

- Add shipped_quantity field to OrderItem for tracking partial fulfillment
- Add partially_shipped order status for orders with partial shipments
- Add fulfill_item method for shipping individual items with quantities
- Add get_shipment_status method for detailed shipment tracking
- Add vendor API endpoints for partial shipment operations:
  - GET /orders/{id}/shipment-status - Get item-level shipment status
  - POST /orders/{id}/items/{item_id}/ship - Ship specific item quantity
- Automatic status updates: partially_shipped when some items shipped,
  shipped when all items fully shipped
- Migration to add shipped_quantity column with upgrade for existing data
- Update documentation with partial shipment usage examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

alembic/versions/p4d5e6f7a8b9_add_shipped_quantity_to_order_items.py

@@ -0,0 +1,39 @@
+# alembic/versions/p4d5e6f7a8b9_add_shipped_quantity_to_order_items.py
+"""Add shipped_quantity to order_items for partial shipments.
+
+Revision ID: p4d5e6f7a8b9
+Revises: o3c4d5e6f7a8
+Create Date: 2026-01-01 12:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'p4d5e6f7a8b9'
+down_revision: Union[str, None] = 'o3c4d5e6f7a8'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add shipped_quantity column to order_items
+    op.add_column(
+        'order_items',
+        sa.Column('shipped_quantity', sa.Integer(), nullable=False, server_default='0')
+    )
+
+    # Set shipped_quantity = quantity for already fulfilled items
+    # This handles existing data where inventory_fulfilled is True
+    op.execute("""
+        UPDATE order_items
+        SET shipped_quantity = quantity
+        WHERE inventory_fulfilled = 1
+    """)
+
+
+def downgrade() -> None:
+    op.drop_column('order_items', 'shipped_quantity')