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/pages/JobDetail.tsx

@@ -80,7 +80,7 @@ export default function JobDetail() {
     try {
       const j = await api<Job>(`/api/jobs/${id}`);
       setJob(j);
-      const pairs = await api<SyncPair[]>('/api/sync-pairs');
+      const pairs = (await api<SyncPair[]>('/api/sync-pairs')) ?? [];
       const p = pairs.find((sp: SyncPair) => sp.id === j.sync_pair_id);
       setPair(p || null);
     } catch {
@@ -91,9 +91,9 @@ export default function JobDetail() {
 
   async function loadLogs(offset: number) {
     try {
-      const ls = await api<LogLine[]>(
+      const ls = (await api<LogLine[]>(
         `/api/jobs/${id}/log?offset=${offset}&limit=1000`
-      );
+      )) ?? [];
       if (offset === 0) {
         setLogs(ls);
       } else {