fix(translate): datasource change not persisted on save + preview column cleanup

Root cause: saveJob() used stale sourceDatasourceId (set once on load)
instead of live datasourceId (updated by ConfigTabForm via $bindable).
Since sourceDatasourceId was always truthy, the || fallback to datasourceId
never triggered — the old datasource ID was always sent to PUT.

Fixes:
- Removed dead sourceDatasourceId atom; saveJob() uses datasourceId directly
- Bound sourceTable via $bindable through ConfigTabForm; updated on select
- loadDatasourceColumns() syncs databaseDialect from Superset columns API
- saveJob() sends undefined for database_dialect="unknown" to force re-detect
- Backend: added direct_db+connection_id validation on update (mirroring create)
- Removed redundant "Язык ист." column from TranslationPreview table
- Removed unused getDetectedLang function

Tests:
- TranslationJobModel: datasourceId save mapping, dialect sync, unknown→undefined
- TranslateJobService: reject direct_db without connection_id, preserve existing

Verified: browser — translate_cross datasource persisted after save+reload,
dialect detected as "clickhouse", columns loaded (2), translation column preserved.

backend/tests/plugins/translate/test_service.py

@@ -324,6 +324,31 @@ class TestUpdateJob:
         job = await svc.update_job(JOB_ID, payload)
         assert job.target_languages == ["fr"]
 
+    @pytest.mark.asyncio
+    async def test_update_direct_db_without_connection_id_rejected(self, db_session):
+        """direct_db updates require an effective connection_id."""
+        config = MagicMock()
+        svc = TranslateJobService(db_session, config)
+        payload = TranslateJobUpdate(insert_method="direct_db", connection_id=None)
+
+        with pytest.raises(ValueError, match="connection_id is required"):
+            await svc.update_job(JOB_ID, payload)
+
+    @pytest.mark.asyncio
+    async def test_update_direct_db_preserves_existing_connection_id(self, db_session):
+        """direct_db update is valid when the existing job already has connection_id."""
+        config = MagicMock()
+        svc = TranslateJobService(db_session, config)
+        base = svc.get_job(JOB_ID)
+        base.connection_id = "conn-1"
+        db_session.commit()
+
+        payload = TranslateJobUpdate(insert_method="direct_db")
+        job = await svc.update_job(JOB_ID, payload)
+
+        assert job.insert_method == "direct_db"
+        assert job.connection_id == "conn-1"
+
 
 class TestDeleteJob:
     """Verify delete_job."""