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/src/models/translate.py

@@ -205,8 +205,6 @@ class TerminologyDictionary(Base):
     id = Column(String, primary_key=True, default=generate_uuid)
     name = Column(String, nullable=False)
     description = Column(Text, nullable=True)
-    source_dialect = Column(String, nullable=False)
-    target_dialect = Column(String, nullable=False)
     is_active = Column(Boolean, default=True)
     created_by = Column(String, nullable=True)
     created_at = Column(DateTime, default=lambda: datetime.now(UTC))