[nitonitori]

Automatic Netease playback currently aborts when the initial jymaster request returns HTTP400/code=-460, even when the same authenticated standard request succeeds. Add one bounded standard-quality retry for auto; explicit quality requests keep their requested quality and receive a clear error.

The playback resolver, song-URL endpoint and administrator diagnostic endpoint share this handling. Other HTTP failures are reported as sanitized 502 responses; an exhausted risk-control response is a sanitized 503. Cookie handling and the existing paid-track/trial fallback are preserved.

Validation:

• Watched the new failure cases reproduce the original HTTPStatusError before implementing the fix.
• Linux/Python3.11 complete backend regression suite: 80 tests passed.
• Covers HTTP400 and HTTP200 risk responses, retry limit, explicit quality, unrelated errors, normal high-quality success, playback payloads and existing trial fallback.
• Read-only live API checks confirmed that authenticated standard quality succeeds for the reported track; no credentials or signed URLs are included in this contribution.
• No web, voice protocol or dependency changes. Actual audible playback is not asserted by the unit tests.

Closes #20