feat(web): "In library, not followed" section on the Artists page

Adds a section at the bottom of the Artists overview listing artists you own
music by (LibraryItem) but don't follow (WatchedArtist), each with a Follow
action — so you can start monitoring artists already in your library.

- /api/artists GET now also returns `ownedNotFollowed`: distinct LibraryItem
  artist names not matching the watched roster (case-insensitive; the known
  have-via-scan name-match caveat, #18).
- Follow resolves the library name → MBID (/api/mb/artists) then POST /api/artists
  with the canonical MB name (mirrors the Last.fm/discover follow flow); a
  no-match surfaces a toast. Followed names hide immediately via a session set,
  even if the MB canonical name differs from the library name.

web 153 tests (owned-not-followed + case-insensitive match asserted), tsc + build clean.

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

web/src/app/api/artists/route.test.ts

@@ -84,6 +84,20 @@ describe("artists collection API", () => {
     expect(after).toMatchObject({ showAllTypes: true, releaseCount: 2, monitoredCount: 2, haveCount: 2 });
   });
 
+  it("GET lists owned artists that are not followed (case-insensitive)", async () => {
+    // one owned + followed, one owned + not followed, one owned matching a followed name by case
+    await prisma.watchedArtist.create({ data: { mbid: "wf", name: "Followed Artist" } });
+    for (const [artist, album] of [["Followed Artist", "A"], ["Unfollowed Artist", "B"], ["followed artist", "C"]] as const) {
+      await prisma.libraryItem.create({
+        data: { artist, album, path: `/${album}`, source: "scan", format: "FLAC", qualityClass: 2 },
+      });
+    }
+    const body = await (await GET(listReq())).json();
+    expect(body.ownedNotFollowed).toContain("Unfollowed Artist");
+    expect(body.ownedNotFollowed).not.toContain("Followed Artist");
+    expect(body.ownedNotFollowed).not.toContain("followed artist"); // case-insensitive match
+  });
+
   it("GET ?mbid= returns the follow state for a single artist", async () => {
     await prisma.watchedArtist.create({ data: { mbid: "followed-1", name: "Followed" } });
     expect((await (await GET(listReq("?mbid=followed-1"))).json()).followed).toBe(true);

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

@@ -11,14 +11,26 @@ export async function GET(request: Request) {
     return Response.json({ followed });
   }
 
-  const artists = await prisma.watchedArtist.findMany({
-    orderBy: { createdAt: "desc" },
-    include: {
-      releases: {
-        select: { monitored: true, currentQualityClass: true, primaryType: true, secondaryTypes: true },
+  const [artists, libraryArtists] = await Promise.all([
+    prisma.watchedArtist.findMany({
+      orderBy: { createdAt: "desc" },
+      include: {
+        releases: {
+          select: { monitored: true, currentQualityClass: true, primaryType: true, secondaryTypes: true },
+        },
       },
-    },
-  });
+    }),
+    prisma.libraryItem.findMany({ select: { artist: true }, distinct: ["artist"] }),
+  ]);
+
+  // Artists you own music by (LibraryItem) but don't follow yet — name-matched
+  // case-insensitively against the watched roster (the have-via-scan name-match caveat).
+  const followedNames = new Set(artists.map((a) => a.name.trim().toLowerCase()));
+  const ownedNotFollowed = libraryArtists
+    .map((r) => r.artist)
+    .filter((name) => name.trim() && !followedNames.has(name.trim().toLowerCase()))
+    .sort((a, b) => a.localeCompare(b));
+
   return Response.json({
     artists: artists.map((a) => {
       const visible = a.showAllTypes ? a.releases : a.releases.filter(isCoreRelease);
@@ -33,6 +45,7 @@ export async function GET(request: Request) {
         haveCount: visible.filter((r) => r.currentQualityClass !== null).length,
       };
     }),
+    ownedNotFollowed,
   });
 }