feat(discover): add DiscoverySuggestion.albumReason

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/prisma/migrations/20260714213448_add_discovery_album_reason/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "DiscoverySuggestion" ADD COLUMN     "albumReason" TEXT;

web/prisma/schema.prisma

@@ -171,6 +171,10 @@ model DiscoverySuggestion {
   score            Float
   seedCount        Int
   sources          String[]
+  // Why this album is suggested (kind='album' only): "newest" (the artist's newest un-owned
+  // release), "popular" (their most-played per Last.fm), or "newest,popular" when they're the
+  // same release. Null for artist suggestions / rows from before this column existed.
+  albumReason      String?
   status           SuggestionStatus @default(pending)
   dedupeKey        String           @unique
   createdAt        DateTime         @default(now())