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> {

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 {

web/src/pages/JobHistory.tsx

@@ -60,7 +60,7 @@ export default function JobHistory() {
       const totalCount = res.headers.get('X-Total-Count');
       if (totalCount) setTotal(Number(totalCount));
       const data = await res.json();
-      setJobs(data);
+      setJobs(Array.isArray(data) ? data : []);
     } catch {
     } finally {
       setLoading(false);