feat(metadata): write source URL into dedicated tag (#605)

[SnitchTeam]

Summary

Adds the originating source URL of each download to a dedicated metadata tag in addition to the existing COMMENT fallback, so library managers (MusicBrainz Picard, Beets, foobar2000, etc.) can read where the file came from.

Changes

• FLAC: write Vorbis SOURCE tag when metadata.URL is non-empty.
• MP3: write the standard ID3v2 WOAS (Official audio source webpage) URL frame, plus a TXXX user-defined frame with description SOURCE for maximum tagger compatibility (Picard/Beets).
• M4A: pass -metadata source=<url> to ffmpeg.

The existing COMMENT/COMM/comment fallback behavior (URL written into the comment when no explicit comment is set) is preserved, so users relying on it are not affected. The new source tag is only written when metadata.URL is non-empty.

Notes

• The bogem/id3v2 library does not expose a typed URL link frame, so WOAS is written via AddFrame("WOAS", id3v2.UnknownFrame{Body: []byte(url)}), which produces a spec-compliant URL link frame body (raw URL bytes, no encoding byte).
• Diff is minimal (3 small additions in backend/metadata.go).

Test

• go build ./... ✅
• go vet ./backend/... ✅

Closes #605