Fix empty-body JSON error on body-less admin requests

The shared cloud req() helper set content-type: application/json on every
request, so body-less DELETE/revoke/rotate calls tripped Fastify's
"Body cannot be empty" 400. Only send the header when there's a body, and
(defensively) make the server parse an empty application/json body as
undefined instead of erroring.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/cloud/campaigns.ts

@@ -16,7 +16,11 @@ export interface CloudCharInfo { id: string; name: string; ownerUserId: string;
 export async function req<T>(path: string, init?: RequestInit): Promise<T> {
   let res: Response;
   try {
-    res = await fetch(`${base()}${path}`, { ...init, headers: { 'content-type': 'application/json', ...authHeaders(), ...(init?.headers ?? {}) } });
+    // Only declare a JSON content-type when we actually send a body — Fastify
+    // rejects an empty body that claims content-type: application/json, which
+    // broke the body-less DELETE/revoke/rotate calls.
+    const jsonType = init?.body != null ? { 'content-type': 'application/json' } : {};
+    res = await fetch(`${base()}${path}`, { ...init, headers: { ...jsonType, ...authHeaders(), ...(init?.headers ?? {}) } });
   } catch {
     throw new CloudError('Could not reach the server.');
   }