feat(metadata): optional 🅴 suffix on explicit track titles (#637)

[SnitchTeam]

Adds an opt-in setting Append 🅴 to explicit track titles (key appendExplicitTag, default off).

When enabled, tracks flagged as explicit on Spotify will get the U+1F174 mark (🅴) appended to:

• the saved file name
• the embedded TITLE tag (FLAC / MP3 / M4A)

Existing users see no change because the setting defaults to off, and the suffix is only added when both the setting is on and the track's is_explicit flag is true. The helper also avoids appending twice if the title already ends with 🅴.

Test

• Toggle the setting in Settings → download an explicit track → the file is saved as Track 🅴 - Artist.flac and the embedded title contains the 🅴 mark.
• With the setting off, behavior is unchanged.
• go build ./... and go vet ./backend/... pass.

Closes #637