fix: include language parameter in marketplace import

Bug: Language selector worked on UI but import always used 'en'

Root causes:
1. Frontend: startImport() was not including language in API payload
2. Backend: language was not stored in import job database record
3. Backend: language was not returned in API response models

Fixes:
- Add language to payload in marketplace.js startImport()
- Add language column to MarketplaceImportJob model
- Store language when creating import job in service
- Include language in both response model converters
- Add database migration for language column

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

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

static/admin/js/marketplace.js

@@ -258,7 +258,8 @@ function adminMarketplace() {
                     vendor_id: parseInt(this.importForm.vendor_id),
                     source_url: this.importForm.csv_url,
                     marketplace: this.importForm.marketplace,
-                    batch_size: this.importForm.batch_size
+                    batch_size: this.importForm.batch_size,
+                    language: this.importForm.language  // Include selected language
                 };
 
                 adminMarketplaceLog.info('Starting import:', payload);