fix: improve fetch_remote error handling and diagnostics

- Add _log.warning() to bare except clause in fetch_remote so unexpected
  errors (JSON parse failures, AttributeError, etc.) leave a diagnostic
  trail instead of silently surfacing as 'unreachable'
- Map HTTPStatusError (non-401/403 HTTP errors like 500, 503, 429) to
  'unreachable' instead of 'auth_failed' — semantically correct since
  these are connectivity/server errors, not authentication failures
- Fix stale task comment in test_api.py: task-8 → task-5

muxplex/main.py

@@ -946,12 +946,13 @@ async def federation_sessions(request: Request) -> list[dict]:
         except httpx.HTTPStatusError:
             return [
                 {
-                    "status": "auth_failed",
+                    "status": "unreachable",
                     "remoteId": remote_id,
                     "deviceName": remote_name,
                 }
             ]
-        except Exception:
+        except Exception as exc:
+            _log.warning("Unexpected error fetching remote %s: %s", url, exc)
             return [
                 {
                     "status": "unreachable",