fix: remove dead code and security issue from code review

- Remove unused _get_pypi_upstream_sources_cached function (never called)
- Remove unused CacheService import and get_cache helper
- Remove unused cache parameter from pypi_download_file
- Fix asyncio.get_event_loop() deprecation - use get_running_loop()
- Note: The caching implementation was incomplete but the other
  performance improvements (connection pooling, batch DB ops) remain

backend/app/http_client.py

@@ -150,7 +150,7 @@ class HttpClientManager:
         if not self._executor:
             raise RuntimeError("HttpClientManager not started. Call startup() first.")
 
-        loop = asyncio.get_event_loop()
+        loop = asyncio.get_running_loop()
         return await loop.run_in_executor(self._executor, func, *args)
 
     @property