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/api/routes/settings.py

@@ -128,6 +128,22 @@ async def get_features(
 # #endregion get_features
 
 
+# #region get_allowed_languages [C:1] [TYPE Function]
+# @BRIEF Public endpoint returning allowed translation languages for language dropdowns.
+# @RATIONALE  No auth required — needed by dictionary page, job config, etc. everywhere
+#            language selects appear. Non-admin users still need the list.
+# @PRE  Config manager is available.
+# @POST  Returns list of BCP-47 language codes.
+@router.get("/allowed-languages")
+async def get_allowed_languages(
+    config_manager: ConfigManager = Depends(get_config_manager),
+):
+    return config_manager.get_config().settings.allowed_languages
+
+
+# #endregion get_allowed_languages
+
+
 # #region update_global_settings [C:2] [TYPE Function]
 # @BRIEF Updates global application settings.
 # @PRE      New settings are provided.
@@ -394,6 +410,7 @@ class ConsolidatedSettingsResponse(BaseModel):
     notifications: dict = {}
     features: dict = {}
     app_timezone: str = "Europe/Moscow"
+    allowed_languages: list[str] = []
 
 
 # #endregion ConsolidatedSettingsResponse
@@ -465,6 +482,7 @@ async def get_consolidated_settings(
             notifications=notifications_payload,
             features=config.settings.features.model_dump(),
             app_timezone=config.settings.app_timezone,
+            allowed_languages=config.settings.allowed_languages,
         )
         logger.reflect(
             "Consolidated settings payload assembled",
@@ -537,6 +555,10 @@ async def update_consolidated_settings(
             current_settings.app_timezone = new_tz
             invalidate_timezone_cache()
 
+        # Update allowed_languages if provided
+        if "allowed_languages" in settings_patch:
+            current_settings.allowed_languages = settings_patch["allowed_languages"]
+
         config_manager.update_global_settings(current_settings)
         return {"status": "success", "message": "Settings updated"}