fix: harden transcode cache integrity and cleanup

[JHNY5]

Summary

Prevents serving corrupt or stale transcoded audio from cache.

Problem

The transcode cache integrity check (valid_cache?) only compares tag duration (±2s) via WahWah. This accepts an empty or partially-written cache file (serving a 0-byte audio response) and never evicts stale entries, so a changed bitrate or replaced media can leave permanently-wrong content cached for a song id.

Changes

• app/controllers/transcoded_stream_controller.rb: valid_cache? now rejects missing or zero-length cache files before trusting the duration comparison
• app/models/stream.rb: new Stream#clean_transcode_cache! evicts stale cache entries for a song (bitrate/source changes)
• test/controllers/transcoded_stream_controller_test.rb: regression test for the empty-cache case

Verification

• Live-tested against a booted instance with the patch: an empty cache file is regenerated, stale caches are evicted, normal streaming unchanged
• Existing transcoded-stream tests remain valid (covered by CI)

This is the master-targeted component; a separate PR targets the v3.2.0 release branch where the API-level fixes live.

[aidewoode]

WahWah already handle these check. so no need to do it again.