Group B fixes

src/app/api/tenants/[name]/assignments/route.ts

@@ -128,6 +128,23 @@ export async function POST(
       { status: 500 }
     );
   }
+  // Bug 7 server-side counterpart: personal tenants are sole-owner
+  // by definition. Reject any assignment attempt — this matches the
+  // hidden panel on the detail page and stops a determined client
+  // (or platform user with a legacy unlabeled personal tenant) from
+  // creating spurious rows.
+  if (
+    tenant.metadata.labels?.["pieced.ch/personal"] === "true" ||
+    (!user.isPlatform && user.isPersonal)
+  ) {
+    return NextResponse.json(
+      {
+        error: "Personal tenants do not support additional assignments.",
+        code: "personal_tenant",
+      },
+      { status: 403 }
+    );
+  }
 
   const body = await req.json().catch(() => null);
   const parsed = assignSchema.safeParse(body);