Fix: defensive frontend + panic recovery logging middleware

Frontend:
- JobDetail loadLogs: fallback to [] when API returns null
- JobDetail loadJob: pairs ?? [] guard on /api/sync-pairs 500
- JobHistory: Array.isArray guard on job list response
- api client: return undefined for null body instead of throwing

Backend:
- handlers_jobs GetLog: return [] instead of null when no log rows
- router: custom recoverer middleware that logs panics to slog
  with full stack trace, method, and path

web/src/api/client.ts

@@ -5,6 +5,10 @@ interface ApiOptions {
   body?: unknown;
 }
 
+function isArray(v: unknown): v is unknown[] {
+  return Array.isArray(v);
+}
+
 export async function api<T>(path: string, opts: ApiOptions = {}): Promise<T> {
   const { method = 'GET', body } = opts;
   const res = await fetch(`${BASE}${path}`, {
@@ -18,7 +22,10 @@ export async function api<T>(path: string, opts: ApiOptions = {}): Promise<T> {
     throw new Error((err as { error?: string }).error || 'Request failed');
   }
   if (res.status === 204) return undefined as T;
-  return res.json();
+  const data = await res.json().catch(() => null);
+  if (data === null) return undefined as T;
+  if (isArray(data) && !data.length && data[0] === undefined) return [] as unknown as T;
+  return data as T;
 }
 
 export async function apiRaw(path: string): Promise<Response> {