Groundwork for admin-created, per-profile-switchable credential sets ("pills")
across auth services (Spotify/Tidal/Deezer/Qobuz/Plex/Jellyfin/Navidrome).
Strictly additive and dormant — nothing reads it at runtime yet, so zero
behaviour change for existing installs.
- core/credentials/store.py: pure service registry + payload validation +
stale-safe active-set selection (pick_active_credential falls back to None
when a selected set was deleted, so a profile never breaks).
- migration service_credentials_v1: two new tables — service_credentials
(admin-created named sets; payload Fernet-encrypted at rest) and
profile_service_credentials (each profile's selected set per service).
- MusicDatabase CRUD: create/update/delete/list/get_service_credential
(list never returns the payload; get decrypts for the resolver), plus
set/get_profile_service_credential and resolve_profile_service_credential
(returns the profile's active payload or None → caller uses global default).
Tests: 12 — pure validation + stale-safe selection, and real-temp-DB storage
proving encryption round-trips, payload never lists, dup(service,label)
rejected, per-profile/per-service resolution, and delete clearing dangling
selections to a clean fallback. 95 migration/DB tests still pass.
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