Spotify provider

Spotify is a provider adapter, not a bootstrap requirement. JamRelay can start with no Spotify configuration and remains healthy with zero providers.

Configuration requires SPOTIFY_CLIENT_ID, SPOTIFY_CLIENT_SECRET, and SPOTIFY_REDIRECT_URI together. SPOTIFY_MARKET defaults to PL and SPOTIFY_READ_CONCURRENCY defaults to 12. Authorization is Spotify OAuth; the resulting credentials are held in the encrypted provider credential store.

The adapter supports identity, catalog/search, playlist reads and writes, library operations, playback controls/devices, and recently played history, subject to Spotify account scopes and API availability. Playlist writes require an explicit or unambiguous connection_id; destructive operations use the snapshot/verify/undo safety path. Playback chapter tools use Spotify's playlist context and an offset; Spotify controls what happens after the selected range.

Existing Spotify data can be represented in canonical tracks and provider mappings. The active credential path is PROVIDER_CREDENTIAL_STORE_PATH; do not place tokens in documentation or source-controlled files.