fix: remove incompatible schema_migrations tracking from migrations 012-013

The db_init.py system uses (filename, applied_at) schema, not (version, description).
Removing manual INSERT statements - db_init.py handles tracking automatically.

backend/migrations/012_exercise_training_characters_and_trainer_contexts.sql

@@ -107,14 +107,6 @@ BEGIN
 END $seed$;
 
 
--- ============================================================================
--- MIGRATION TRACKING
--- ============================================================================
-
-INSERT INTO schema_migrations (version, description)
-VALUES (12, 'exercise_training_characters + trainer_contexts')
-ON CONFLICT (version) DO NOTHING;
-
 RAISE NOTICE 'Migration 012 completed successfully';
 
 END $$;

backend/migrations/013_training_types_focus_area.sql

@@ -128,14 +128,6 @@ BEGIN
 END $constraint$;
 
 
--- ============================================================================
--- MIGRATION TRACKING
--- ============================================================================
-
-INSERT INTO schema_migrations (version, description)
-VALUES (13, 'training_types focus_area hierarchy')
-ON CONFLICT (version) DO NOTHING;
-
 RAISE NOTICE 'Migration 013 completed successfully';
 
 END $$;