#356 Document playlist browsing and organization fan ownership

Document organization-owned fan contacts and the playlist song browser so release operators can configure destinations and understand fan attribution across artists.

• Add the workspace fan endpoint, contact/artist relationship schemas, and additive contact IDs on existing fan/activity responses.
• Explain separate organization ownership, exact provider identity matching, playlist selection/fallback, and migration-before-API rollout order.
• Validation: OpenAPI JSON, navigation targets and local schema references checked; diff whitespace check passes. Documentation describes the coordinated feature and does not claim production activation.

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Summary by cubic

Documents the organization-owned fan contact model and the playlist song browser so release operators can configure destinations and understand fan attribution across artists.

• Adds the GET /api/players/fans workspace endpoint, contact/artist relationship schemas, and contact_id on existing fan and activity responses.
• Explains exact provider identity matching, independent per-workspace ownership, and the migration-before-API rollout order.
• Covers playlist selection from a paginated song list, highlight from Spotify playback state, and behavior for unavailable or local entries.

Migration

• Apply 20261010100000_player_organization_contacts.sql before deploying API consumers; the browser playlist change can ship independently.