chore: remove legacy SQLite migration script + fix stale comments

- Delete migrate_sqlite_to_postgres.py (one-time script, no longer needed)
- Fix @INVARIANT in mappings.py: SQLite → PostgreSQL
- Remove deprecated source_language column from TranslationJob model
  + Alembic migration 8dd0a93af539 to drop column if exists
- Remove source_language from TranslateJobResponse schema

Fixes recurring 'source_language does not exist' scheduler error

backend/src/api/routes/mappings.py

@@ -6,7 +6,7 @@
 # @RELATION DEPENDS_ON -> [DatabaseModule]
 # @RELATION DEPENDS_ON -> [mapping_service]
 #
-# @INVARIANT: Mappings are persisted in the SQLite database.
+#
 
 
 from fastapi import APIRouter, Depends, HTTPException