feat: add metadata validation and scan acceptance support

src/test/resources/audio-fixtures/README.md

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+Test audio fixture guidance
+
+The current unit tests avoid committing real audio binaries and instead use `byte[]` plus a mocked `AudioTagExtractor`.
+
+Recommended real-file fixtures if you later want integration tests:
+
+1. `valid-square-cover.flac`
+   - Tags: `title`, `artist`, `album_artist`, `album`, `track=01/12`
+   - Embedded PNG cover `320x320`
+   - UTF-8 encoded text
+
+2. `missing-cover.mp3`
+   - Same core tags as above
+   - No embedded artwork
+   - Expected validator result: `COVER_MISSING`
+
+3. `bad-track.m4a`
+   - Track stored as `1` instead of `01/12`
+   - Expected validator result: `INVALID_FORMAT`
+
+4. `garbled-title.ogg`
+   - Title intentionally stored with broken encoding bytes so it decodes to `Bad <20> Title`
+   - Expected reader encoding result: `UNKNOWN`
+
+To generate those fixtures quickly, ffmpeg plus a tagging tool is enough:
+
+```bash
+ffmpeg -f lavfi -i sine=frequency=440:duration=1 -c:a flac valid.flac
+```
+
+Then embed tags and artwork with a dedicated tag editor, or continue mocking `AudioTagExtractor` in unit tests for faster coverage.