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/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);