feat(backend): drop dictionary dialect columns, add allowed_languages

- Removes source_dialect and target_dialect from TerminologyDictionary model,
  schemas, routes, helper serialization, and all tests
- Adds allowed_languages config field (BCP-47 language codes) to GlobalSettings
  with validation, consolidated settings response, and API endpoint
- Adds alembic migration f1a2b3c4d5e6 to drop the two columns

backend/tests/test_translate_corrections.py

@@ -105,7 +105,7 @@ def client(mock_api_deps):
 def test_submit_correction_creates_entry(db_session):
     """Test that submitting a correction creates a new entry."""
     dict_obj = DictionaryManager.create_dictionary(
-        db_session, name="Test Dict", source_dialect="en", target_dialect="ru",
+        db_session, name="Test Dict",
         created_by="testuser",
     )
 
@@ -138,7 +138,7 @@ def test_submit_correction_creates_entry(db_session):
 def test_submit_correction_conflict_detected(db_session):
     """Test that conflict is detected when entry already exists."""
     dict_obj = DictionaryManager.create_dictionary(
-        db_session, name="Dict", source_dialect="en", target_dialect="ru",
+        db_session, name="Dict",
     )
 
     # Create initial entry
@@ -165,7 +165,7 @@ def test_submit_correction_conflict_detected(db_session):
 def test_submit_correction_overwrite(db_session):
     """Test that correction overwrites existing entry."""
     dict_obj = DictionaryManager.create_dictionary(
-        db_session, name="Dict", source_dialect="en", target_dialect="ru",
+        db_session, name="Dict",
     )
 
     DictionaryManager.add_entry(db_session, dict_obj.id, "hello", "privet")
@@ -190,7 +190,7 @@ def test_submit_correction_overwrite(db_session):
 def test_bulk_corrections_atomic(db_session):
     """Test that bulk corrections are applied atomically."""
     dict_obj = DictionaryManager.create_dictionary(
-        db_session, name="Bulk Dict", source_dialect="en", target_dialect="ru",
+        db_session, name="Bulk Dict",
     )
 
     corrections = [
@@ -233,7 +233,7 @@ def test_api_bulk_corrections(client, mock_api_deps):
     """Test bulk corrections endpoint."""
     session = mock_api_deps["session"]
     dict_obj = DictionaryManager.create_dictionary(
-        session, name="API Dict", source_dialect="en", target_dialect="ru",
+        session, name="API Dict",
     )
 
     response = client.post("/api/translate/corrections/bulk", json={