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>

models/database/marketplace_import_job.py

@@ -64,6 +64,7 @@ class MarketplaceImportJob(Base, TimestampMixin):
     # Import configuration
     marketplace = Column(String, nullable=False, index=True, default="Letzshop")
     source_url = Column(String, nullable=False)
+    language = Column(String(5), nullable=False, default="en")  # Language for translations
 
     # Status tracking
     status = Column(