feat(photo-upload): Logging fuer Upload-Parse-Fehler

Der bisherige Endpoint verschluckte den formData()-Fehler mit einem
generischen "Multipart erwartet" — wir wissen nicht, warum Chrome auf
dem Tablet scheitert. Jetzt wird beim Fehler Content-Type, -Length und
User-Agent geloggt, plus die konkrete Error-Message in der Response.
Kein Foto-Inhalt im Log.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/routes/api/recipes/extract-from-photo/+server.ts

@@ -45,16 +45,38 @@ export const POST: RequestHandler = async ({ request, getClientAddress }) => {
     );
   }
 
+  // Header-Snapshot fuer Diagnose beim Upload-Parse-Fehler. Wir loggen
+  // Content-Type, -Length und User-Agent — nichts, was Inhalt verraet.
+  const contentType = request.headers.get('content-type') ?? '(missing)';
+  const contentLength = request.headers.get('content-length') ?? '(missing)';
+  const userAgent = request.headers.get('user-agent')?.slice(0, 120) ?? '(missing)';
+
   let form: FormData;
   try {
     form = await request.formData();
-  } catch {
-    return errJson(400, 'BAD_REQUEST', 'Multipart body erwartet.');
+  } catch (e) {
+    const err = e as Error;
+    console.warn(
+      `[extract-from-photo] formData() failed: name=${err.name} msg=${err.message} ` +
+        `ct="${contentType}" len=${contentLength} ua="${userAgent}"`
+    );
+    return errJson(
+      400,
+      'BAD_REQUEST',
+      `Upload konnte nicht gelesen werden (${err.name}: ${err.message}).`
+    );
   }
   const photo = form.get('photo');
   if (!(photo instanceof Blob)) {
+    console.warn(
+      `[extract-from-photo] photo field missing or not a Blob. ct="${contentType}" ` +
+        `len=${contentLength} fields=${[...form.keys()].join(',')}`
+    );
     return errJson(400, 'BAD_REQUEST', 'Feld "photo" fehlt.');
   }
+  console.info(
+    `[extract-from-photo] received photo size=${photo.size} mime="${photo.type}" ua="${userAgent}"`
+  );
   if (photo.size > MAX_BYTES) {
     return errJson(
       413,