feat: add POST endpoint for admin marketplace import jobs

Add POST /api/v1/admin/marketplace-import-jobs endpoint to allow
admins to create import jobs for any vendor.

Changes:
- Add AdminMarketplaceImportJobRequest schema with vendor_id field
- Add create_marketplace_import_job endpoint in admin/marketplace.py
- Make vendor_code and vendor_name optional in response model
  to handle edge cases where vendor relationship may not be loaded

This fixes the 405 Method Not Allowed error when trying to import
products from the admin marketplace page.

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

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

models/schema/marketplace_import_job.py

@@ -29,6 +29,33 @@ class MarketplaceImportJobRequest(BaseModel):
         return v.strip()
 
 
+class AdminMarketplaceImportJobRequest(BaseModel):
+    """Request schema for admin-triggered marketplace import.
+
+    Includes vendor_id since admin can import for any vendor.
+    """
+
+    vendor_id: int = Field(..., description="Vendor ID to import products for")
+    source_url: str = Field(..., description="URL to CSV file from marketplace")
+    marketplace: str = Field(default="Letzshop", description="Marketplace name")
+    batch_size: int | None = Field(
+        1000, description="Processing batch size", ge=100, le=10000
+    )
+
+    @field_validator("source_url")
+    @classmethod
+    def validate_url(cls, v):
+        # Basic URL security validation
+        if not v.startswith(("http://", "https://")):
+            raise ValueError("URL must start with http:// or https://")
+        return v.strip()
+
+    @field_validator("marketplace")
+    @classmethod
+    def validate_marketplace(cls, v):
+        return v.strip()
+
+
 class MarketplaceImportJobResponse(BaseModel):
     """Response schema for marketplace import job."""
 
@@ -36,8 +63,8 @@ class MarketplaceImportJobResponse(BaseModel):
 
     job_id: int
     vendor_id: int
-    vendor_code: str  # Populated from vendor relationship
-    vendor_name: str  # Populated from vendor relationship
+    vendor_code: str | None = None  # Populated from vendor relationship
+    vendor_name: str | None = None  # Populated from vendor relationship
     marketplace: str
     source_url: str
     status: str