fix(custom-metrics): say "name taken" only for a name clash

The form read every 409 as "a metric with this name already exists".
Now that it keeps one idempotency key across taps, a second tap while
the first create is still running also answers 409, and the form told
the person their name was taken. The create and rename routes send
`customMetric.name_taken` with the clash, and the form matches the code.

src/app/api/custom-metrics/[id]/__tests__/route.test.ts

@@ -225,6 +225,7 @@ describe("PATCH /api/custom-metrics/[id]", () => {
 
     const res = await PATCH(patchReq({ name: "Other" }), params("cm-1"));
     expect(res.status).toBe(409);
+    expect((await res.json()).meta.errorCode).toBe("customMetric.name_taken");
     expect(prisma.customMetric.update).not.toHaveBeenCalled();
   });
 });

src/app/api/custom-metrics/[id]/route.ts

@@ -111,7 +111,9 @@ export const PATCH = apiHandler(
         select: { id: true },
       });
       if (clash) {
-        return apiError("A custom metric with this name already exists", 409);
+        return apiError("A custom metric with this name already exists", 409, {
+          errorCode: "customMetric.name_taken",
+        });
       }
     }
 

src/app/api/custom-metrics/__tests__/route.test.ts

@@ -134,6 +134,9 @@ describe("POST /api/custom-metrics", () => {
 
     const res = await POST(postReq({ name: "Grip strength", unit: "kg" }));
     expect(res.status).toBe(409);
+    // The form names the clash by this code: a 409 without it can be the
+    // first save still running under the same key.
+    expect((await res.json()).meta.errorCode).toBe("customMetric.name_taken");
     expect(prisma.customMetric.create).not.toHaveBeenCalled();
   });
 

src/app/api/custom-metrics/route.ts

@@ -100,7 +100,9 @@ async function postCustomMetric(request: NextRequest) {
     select: { id: true, deletedAt: true },
   });
   if (existing && existing.deletedAt === null) {
-    return apiError("A custom metric with this name already exists", 409);
+    return apiError("A custom metric with this name already exists", 409, {
+      errorCode: "customMetric.name_taken",
+    });
   }
 
   // Field-by-field assignment — never spread `parsed.data`.