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

@@ -50,23 +50,28 @@ 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
-  if (releases.length > 0) {
+  // a followed artist with no releases (which the 409 guard above would then wedge).
-    await prisma.monitoredRelease.createMany({
+  const artist = await prisma.$transaction(async (tx) => {
-      data: releases.map((r) => ({
+    const created = await tx.watchedArtist.create({ data: { mbid, name: name.trim() } });
-        watchedArtistId: artist.id,
+    if (releases.length > 0) {
-        artistMbid: mbid,
+      await tx.monitoredRelease.createMany({
-        artistName: name.trim(),
+        data: releases.map((r) => ({
-        rgMbid: r.rgMbid,
+          watchedArtistId: created.id,
-        album: r.title,
+          artistMbid: mbid,
-        primaryType: r.primaryType,
+          artistName: name.trim(),
-        secondaryTypes: r.secondaryTypes,
+          rgMbid: r.rgMbid,
-        firstReleaseDate: r.firstReleaseDate,
+          album: r.title,
-        monitored: false,
+          primaryType: r.primaryType,
-      })),
+          secondaryTypes: r.secondaryTypes,
-      skipDuplicates: true,
+          firstReleaseDate: r.firstReleaseDate,
-    });
+          monitored: false,
-  }
+        })),
+        skipDuplicates: true,
+      });
+    }
+    return created;
+  });
   return Response.json(
     { id: artist.id, mbid: artist.mbid, name: artist.name, releaseCount: releases.length },
     { status: 201 },