fix(aria2): disable fsync and relax RPC timeouts to prevent aria2 freeze on large files

aria2 default --fsync=true calls fsync() after each write, which blocks
the entire single-threaded process on large files under Docker overlay.
Add --fsync=false to eliminate this blocking source.

Relax aiohttp session timeout: total=30 → sock_connect=10, sock_read=60
so that transient I/O delays don't cut off legitimate tellStatus RPCs.

Increase retry params (4 attempts, 3s delay) to give aria2 more recovery
time when blocked on synchronous I/O.

tests/services/test_aria2_downloader.py

@@ -410,7 +410,7 @@ async def test_get_status_with_retry_raises_after_all_retries_exhausted(monkeypa
         await downloader._get_status_with_retry("dummy")
 
     msg = str(exc_info.value)
-    assert "after 3 attempts" in msg
+    assert "after 4 attempts" in msg
     assert "connection reset" in msg