fix(backend): apply the large-body limit to every spelling Express routes

req.path was compared to the canonical '/api/voice/extract' only, but
Express routes case-insensitively and ignores a trailing slash by default.
'/api/voice/extract/' therefore reached the controller with the 100 kb
parser, and 413'd every recording past ~20 seconds — a failure that reads
as a broken microphone rather than a routing detail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/common/body-parsers.ts

@@ -24,12 +24,22 @@ export const VOICE_BODY_LIMIT = '10mb';
  * the endpoint rejected every real recording with a 500. One explicit branch has no such
  * coupling, and is covered by body-parsers.spec.ts.
  */
+/**
+ * Express routes case-insensitively and ignores a trailing slash unless configured
+ * otherwise, so `/API/Voice/Extract/` reaches the same controller. Matching only the
+ * canonical spelling would hand those requests the 100 kb parser and 413 every real
+ * recording — a failure that looks like a broken microphone, not a routing detail.
+ */
+function isVoiceExtractPath(path: string): boolean {
+  return path.toLowerCase().replace(/\/+$/, '') === VOICE_EXTRACT_PATH;
+}
+
 export function createJsonBodyParser(): RequestHandler {
   const voiceParser = json({ limit: VOICE_BODY_LIMIT });
   const defaultParser = json();
 
   return (req: Request, res: Response, next: NextFunction) =>
-    req.path === VOICE_EXTRACT_PATH
+    isVoiceExtractPath(req.path)
       ? voiceParser(req, res, next)
       : defaultParser(req, res, next);
 }