Use osu API rate limit headers

[cmyui]

Summary

• consume osu API X-Ratelimit-Remaining headers in the shared backoff path
• open the v2 circuit after a successful response reports zero remaining quota, so the next request fails locally instead of becoming a 429
• use X-Ratelimit-Reset as a cooldown fallback when Retry-After is absent
• wire the header feedback into v2 beatmap/beatmapset/search responses and OAuth token refreshes

Upstream context

• osu-web's API route group uses its custom ThrottleRequests::getApiThrottle() middleware for /api/v2: https://github.com/ppy/osu-web/blob/master/routes/web.php#L442-L443
• osu-web's throttle middleware adds the standard throttle headers after requests: https://github.com/ppy/osu-web/blob/master/app/Http/Middleware/ThrottleRequests.php#L24-L45
• osu-web tests assert X-Ratelimit-Limit and X-Ratelimit-Remaining: https://github.com/ppy/osu-web/blob/master/tests/Middleware/ThrottleRequestsTest.php#L24-L39

Tests

• python3 -m unittest discover tests
• python3 -m py_compile app/adapters/osu_api_backoff.py app/adapters/osu_api_v2/api.py app/oauth.py tests/test_osu_api_backoff.py
• python3 -m mypy app/adapters/osu_api_backoff.py app/adapters/osu_api_v2/api.py app/oauth.py