feat(Apple Music): Add catalog API provider for streaming-only releases

[dwei30]

Summary

• Adds a separate Apple Music provider that looks up music.apple.com albums via the catalog API (/v1/catalog/{storefront}/albums/{id}), including streaming-only releases that the iTunes Search API returns with no tracks (and often no GTIN from artwork URLs).
• Prefers the official Apple Music API (api.music.apple.com) when HARMONY_APPLE_MUSIC_TOKEN is set. For self-hosted instances, unofficial AMP (amp-api.music.apple.com) is available via HARMONY_APPLE_MUSIC_AMP_TOKEN or HARMONY_APPLE_MUSIC_SCRAPE=true.
• iTunes is unchanged (Search API only). Apple Music is registered only when one of those env vars is set, and only then does it claim music.apple.com URLs. itunes.apple.com stays on iTunes. Without credentials, Harmony behaves as today.

Auth (pick one; first match wins):

1. HARMONY_APPLE_MUSIC_TOKEN — official developer JWT
2. HARMONY_APPLE_MUSIC_AMP_TOKEN — unofficial AMP JWT
3. HARMONY_APPLE_MUSIC_SCRAPE — scrape a MusicKit JWT from music.apple.com (self-hosted)

Needs testing with an official Apple Music API token as I don't have access to one.

.env

Copy from .env.example. You only need one of these; leave the others blank.

# 1. Official Apple Music API (preferred). MusicKit developer JWT.
HARMONY_APPLE_MUSIC_TOKEN=
# 2. Unofficial AMP JWT, used only if HARMONY_APPLE_MUSIC_TOKEN is unset.
HARMONY_APPLE_MUSIC_AMP_TOKEN=
# 3. Scrape a MusicKit JWT from music.apple.com when no token is set (self-hosted only).
# Set true or false.
HARMONY_APPLE_MUSIC_SCRAPE=

Local / self-hosted without an Apple Developer key:

HARMONY_APPLE_MUSIC_SCRAPE=true

Sources

• Related: iTunes API returns less tracks than Apple Music API
• AMP catalog / MusicKit token approach: ToadKing/apple-music-barcode-isrc