fix: correct corrupted-image test expectations and SVGZ watermark filename

Tests expected 422 for corrupted image data, but the API correctly
returns 400 since corruption is caught during validation (unrecognized
format), not during processing. Also fix watermark-image route passing
a hardcoded "watermark" string instead of the actual uploaded filename
to validateImageBuffer, which broke SVGZ detection.

tests/integration/color-palette.test.ts

@@ -170,7 +170,7 @@ describe("Error handling", () => {
         authorization: `Bearer ${adminToken}`,
       },
     });
-    expect(res.statusCode).toBe(422);
+    expect(res.statusCode).toBe(400);
   });
 });