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>

app/services/marketplace_import_job_service.py

@@ -48,6 +48,7 @@ class MarketplaceImportJobService:
                 status="pending",
                 source_url=request.source_url,
                 marketplace=request.marketplace,
+                language=request.language,
                 vendor_id=vendor.id,
                 user_id=user.id,
             )
@@ -185,6 +186,7 @@ class MarketplaceImportJobService:
             job_id=job.id,
             status=job.status,
             marketplace=job.marketplace,
+            language=job.language,
             vendor_id=job.vendor_id,
             vendor_code=job.vendor.vendor_code if job.vendor else None,
             vendor_name=job.vendor.name if job.vendor else None,
@@ -208,6 +210,7 @@ class MarketplaceImportJobService:
             job_id=job.id,
             status=job.status,
             marketplace=job.marketplace,
+            language=job.language,
             vendor_id=job.vendor_id,
             vendor_code=job.vendor.vendor_code if job.vendor else None,
             vendor_name=job.vendor.name if job.vendor else None,