fix(aria2): move stderr drain after _wait_until_ready to avoid swallowing startup errors

_drain_stderr and _wait_until_ready both read from the same stderr pipe.
Starting the drain task before _wait_until_ready creates a race where the
drain task consumes aria2's early-exit error message before the startup
waiter can read it, resulting in an empty error message in the logs.

Also confirmed that --fsync does not exist as an aria2 option (exit code
28 = Invalid argument).

py/services/aria2_downloader.py

@@ -520,16 +520,19 @@ class Aria2Downloader:
                 stderr=asyncio.subprocess.PIPE,
             )
 
+            await self._wait_until_ready()
+
             # Drain aria2's stderr in a background task so the pipe buffer
             # never fills up.  If the pipe blocks, aria2 itself freezes and
             # cannot respond to RPC — this was the root cause of the
             # "Failed to query aria2 download status" timeout bug.
+            # Must start AFTER _wait_until_ready to avoid a race where the
+            # drain task consumes aria2's early-exit error message before
+            # _wait_until_ready can read it.
             self._stderr_reader_task = asyncio.create_task(
                 self._drain_stderr()
             )
 
-            await self._wait_until_ready()
-
     def _resolve_executable(self) -> str:
         settings = get_settings_manager()
         configured_path = (settings.get("aria2c_path") or "").strip()