fix(download): return 200 for missing queue items and quiet download-progress 404s

The browser extension's apiFetch treats any 404 as a missing endpoint and
retries the legacy non-/api/lm URL, producing two spurious
'error_middleware - WARNING - API GET ... 404' log lines per occurrence.

- complete_download_in_queue / update_download_queue_status /
  retry_download_from_history: 'not found' is a normal business outcome,
  return 200 + success:false instead of 404 (extension behavior unchanged;
  apiGet ignores the HTTP status)
- error_middleware: downgrade /api/lm/download-progress/ 404s to debug like
  previews - the 404 status itself stays (extension uses it for failure
  detection), only the log level is lowered

py/middleware/error_middleware.py

@@ -46,6 +46,16 @@ async def api_json_error(
         if request.path.startswith("/api/lm/previews") and exc.status == 404:
             logger_method = logger.debug
 
+        # Download-progress 404 is routine too: in-memory tracking is removed
+        # once a download finishes/fails, so the extension's final polls 404.
+        # The extension relies on the 404 status itself (failure detection),
+        # so only the log level is lowered.
+        if (
+            request.path.startswith("/api/lm/download-progress/")
+            and exc.status == 404
+        ):
+            logger_method = logger.debug
+
         logger_method(
             "API %s %s returned HTTP %d: %s",
             request.method,