feat: add media library picker for product images

- Add admin media API endpoints for vendor media management
- Create reusable media_picker_modal macro in modals.html
- Create mediaPickerMixin Alpine.js helper for media selection
- Update product create/edit forms with media picker UI
- Support main image + additional images selection
- Add upload functionality within the picker modal
- Update vendor_product_service to handle additional_images
- Add additional_images field to Pydantic schemas

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

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

app/services/vendor_product_service.py

@@ -286,6 +286,7 @@ class VendorProductService:
             tax_rate_percent=data.get("tax_rate_percent", 17),
             availability=data.get("availability"),
             primary_image_url=data.get("primary_image_url"),
+            additional_images=data.get("additional_images"),
             is_active=data.get("is_active", True),
             is_featured=data.get("is_featured", False),
             is_digital=is_digital,
@@ -399,6 +400,7 @@ class VendorProductService:
             "is_active",
             "is_featured",
             "primary_image_url",
+            "additional_images",
             "supplier",
         ]