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>

alembic/versions/b412e0b49c2e_add_language_column_to_marketplace_.py

@@ -0,0 +1,30 @@
+"""add language column to marketplace_import_jobs
+
+Revision ID: b412e0b49c2e
+Revises: 91d02647efae
+Create Date: 2025-12-13 13:35:46.524893
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'b412e0b49c2e'
+down_revision: Union[str, None] = '91d02647efae'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add language column with default value for existing rows
+    op.add_column(
+        'marketplace_import_jobs',
+        sa.Column('language', sa.String(length=5), nullable=False, server_default='en')
+    )
+
+
+def downgrade() -> None:
+    op.drop_column('marketplace_import_jobs', 'language')