docs(plan): add M5c suggested-additions implementation plan
9 tasks: migration 0012 (artist_similarity_unmatched), ListenBrainz client extension (SimilarArtist.Name), similarity worker extension to persist unmatched MBIDs, recommendation.SuggestArtists service with single CTE, GET /api/discover/suggestions handler, frontend client + types, DiscoverResultCard attribution prop, SuggestionFeed component + /discover integration.
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