fix(artists-api): create artist + discography atomically

Wrap the WatchedArtist.create + MonitoredRelease.createMany in a
$transaction so a mid-write failure can't leave a followed artist with
no releases (a state the 409 duplicate-follow guard would then wedge).

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

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

@@ -50,11 +50,14 @@ export async function POST(request: Request) {
     return Response.json({ error: "MusicBrainz unavailable" }, { status: 502 });
   }
 
-  const artist = await prisma.watchedArtist.create({ data: { mbid, name: name.trim() } });
+  // Create the artist and its discography atomically so a mid-write failure can't leave
+  // a followed artist with no releases (which the 409 guard above would then wedge).
+  const artist = await prisma.$transaction(async (tx) => {
+    const created = await tx.watchedArtist.create({ data: { mbid, name: name.trim() } });
     if (releases.length > 0) {
-    await prisma.monitoredRelease.createMany({
+      await tx.monitoredRelease.createMany({
         data: releases.map((r) => ({
-        watchedArtistId: artist.id,
+          watchedArtistId: created.id,
           artistMbid: mbid,
           artistName: name.trim(),
           rgMbid: r.rgMbid,
@@ -67,6 +70,8 @@ export async function POST(request: Request) {
         skipDuplicates: true,
       });
     }
+    return created;
+  });
   return Response.json(
     { id: artist.id, mbid: artist.mbid, name: artist.name, releaseCount: releases.length },
     { status: 201 },