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>

models/schema/vendor_product.py

@@ -107,6 +107,7 @@ class VendorProductDetail(BaseModel):
     availability_source: str | None = None
     primary_image_url: str | None = None
     primary_image_url_source: str | None = None
+    additional_images: list[str] | None = None
     is_digital: bool | None = None
     product_type: str | None = None
     # Vendor-specific fields
@@ -138,6 +139,10 @@ class VendorProductDetail(BaseModel):
     # Translations
     marketplace_translations: dict | None = None
     vendor_translations: dict | None = None
+    # Convenience fields for UI display
+    title: str | None = None
+    description: str | None = None
+    image_url: str | None = None  # Alias for primary_image_url
     # Timestamps
     created_at: str | None = None
     updated_at: str | None = None
@@ -177,8 +182,9 @@ class VendorProductCreate(BaseModel):
     tax_rate_percent: int | None = 17  # Default Luxembourg VAT
     availability: str | None = None
 
-    # Image
+    # Images
     primary_image_url: str | None = None
+    additional_images: list[str] | None = None
 
     # Status
     is_active: bool = True
@@ -227,6 +233,7 @@ class VendorProductUpdate(BaseModel):
 
     # Images
     primary_image_url: str | None = None
+    additional_images: list[str] | None = None
 
     # Optional supplier info
     supplier: str | None = None