032: fix missing awaits in translate datasource endpoints (#5,6,7)

Three sync functions called async SupersetClient methods without await:
  - get_datasource_columns(): get_dataset_detail + get_database (async)
  - fetch_available_datasources(): get_datasets (async)
  - Route handlers: both calls missing await

Converted to async functions + added awaits. Both endpoints now return
proper data instead of coroutine errors.

backend/src/plugins/translate/service.py

@@ -234,13 +234,13 @@ class TranslateJobService:
 
     # region fetch_available_datasources [TYPE Function]
     # @PURPOSE: List available Superset datasets for translation job creation.
-    def fetch_available_datasources(self, env_id: str, search: str | None = None) -> list:
+    async def fetch_available_datasources(self, env_id: str, search: str | None = None) -> list:
         from ...core.superset_client import SupersetClient
         env = self.config_manager.get_environment(env_id)
         if not env:
             raise ValueError(f"Environment '{env_id}' not found")
         client = SupersetClient(env)
-        _, datasets = client.get_datasets()
+        _, datasets = await client.get_datasets()
         result = []
         for ds in datasets:
             name = ds.get("table_name", "")