feat(discover): group suggestions into artist rows

/api/discover now returns rows[] grouped by artist, each carrying its album
suggestions (with albumReason) + the seed names, ordered by score. Album
suggestions whose artist has no pending artist suggestion still yield a row.

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

web/src/app/api/discover/route.ts

@@ -1,15 +1,35 @@
 import { prisma } from "@/lib/db";
 
+// One Discover row = a suggested artist plus that artist's suggested albums (newest /
+// most-played), since album suggestions are always derived from a surfaced artist. Pending
+// suggestions are grouped by artistMbid so the UI shows a single artist-centric row.
+type Row = {
+  id: string | null; // the artist suggestion id (Follow/Dismiss); null if only albums exist
+  artistMbid: string;
+  artistName: string;
+  score: number;
+  seedCount: number;
+  sources: string[];
+  seeds: string[];
+  albums: {
+    id: string;
+    rgMbid: string | null;
+    album: string | null;
+    primaryType: string | null;
+    secondaryTypes: string[];
+    firstReleaseDate: string | null;
+    albumReason: string | null;
+  }[];
+};
+
 export async function GET() {
   const rows = await prisma.discoverySuggestion.findMany({
     where: { status: "pending" },
     orderBy: [{ score: "desc" }, { createdAt: "asc" }],
   });
 
-  // "Why suggested": the followed artists (seeds) that surfaced each candidate. A suggestion's
-  // candidate is its artistMbid (for both artist and album kinds — the album's artist is what
-  // was found similar). Join the per-seed contributions to the current watched roster so an
-  // unfollowed seed drops out. Highest-scoring seed first.
+  // "Why suggested": the followed artists (seeds) that surfaced each candidate, from the
+  // per-seed contributions joined to the current watched roster (an unfollowed seed drops out).
   const candidateMbids = [...new Set(rows.map((r) => r.artistMbid))];
   const contribs = candidateMbids.length
     ? await prisma.discoverySeedContribution.findMany({
@@ -23,28 +43,57 @@ export async function GET() {
   const seedsByCandidate = new Map<string, string[]>();
   for (const c of contribs) {
     const name = seedNames.get(c.seedMbid);
-    if (!name) continue; // seed no longer followed
+    if (!name) continue;
     const list = seedsByCandidate.get(c.candidateMbid) ?? [];
     if (!list.includes(name)) list.push(name);
     seedsByCandidate.set(c.candidateMbid, list);
   }
 
-  const map = (r: (typeof rows)[number]) => ({
-    id: r.id,
-    artistMbid: r.artistMbid,
-    artistName: r.artistName,
-    rgMbid: r.rgMbid,
-    album: r.album,
-    primaryType: r.primaryType,
-    secondaryTypes: r.secondaryTypes,
-    firstReleaseDate: r.firstReleaseDate,
-    score: r.score,
-    seedCount: r.seedCount,
-    sources: r.sources,
-    seeds: seedsByCandidate.get(r.artistMbid) ?? [],
-  });
-  return Response.json({
-    artists: rows.filter((r) => r.kind === "artist").map(map),
-    albums: rows.filter((r) => r.kind === "album").map(map),
-  });
+  // Group by artist. An artist suggestion sets the row header; album suggestions fill albums[].
+  // An album whose artist has no pending artist suggestion still yields a header (never orphaned).
+  const byArtist = new Map<string, Row>();
+  const ensure = (r: (typeof rows)[number]): Row => {
+    let row = byArtist.get(r.artistMbid);
+    if (!row) {
+      row = {
+        id: null,
+        artistMbid: r.artistMbid,
+        artistName: r.artistName,
+        score: r.score,
+        seedCount: r.seedCount,
+        sources: r.sources,
+        seeds: seedsByCandidate.get(r.artistMbid) ?? [],
+        albums: [],
+      };
+      byArtist.set(r.artistMbid, row);
+    }
+    return row;
+  };
+
+  for (const r of rows) {
+    const row = ensure(r);
+    if (r.kind === "artist") {
+      row.id = r.id;
+      row.score = r.score;
+      row.seedCount = r.seedCount;
+      row.sources = r.sources;
+    } else {
+      row.albums.push({
+        id: r.id,
+        rgMbid: r.rgMbid,
+        album: r.album,
+        primaryType: r.primaryType,
+        secondaryTypes: r.secondaryTypes,
+        firstReleaseDate: r.firstReleaseDate,
+        albumReason: r.albumReason,
+      });
+      row.score = Math.max(row.score, r.score); // album-only row scores by its best album
+    }
+  }
+
+  for (const row of byArtist.values()) {
+    row.albums.sort((a, b) => (b.firstReleaseDate ?? "").localeCompare(a.firstReleaseDate ?? ""));
+  }
+  const out = [...byArtist.values()].sort((a, b) => b.score - a.score);
+  return Response.json({ rows: out });
 }