Fix 10 bugs found in general code review

Backend (app.py / database.py / ai_agent.py):
- [Critique] Autoclose loop: add pg_try_advisory_lock so only one Gunicorn
  worker runs the check per 60s cycle; add random startup jitter
- [Critique] admin_delete_idea: pass consultation_id to
  _update_synthesis_background so the right synthesis is regenerated
- [Majeur] admin_login: return HMAC-signed session token instead of raw
  ADMIN_SECRET; require_admin verifies the signature (TTL 8h)
- [Majeur] bulk_delete: replace str.isdigit() (Unicode-unsafe) with
  try/except int() to prevent crash on Unicode digit characters
- [Majeur] create_consultation: force UTC timezone on naive datetime from
  fromisoformat() to prevent TypeError when comparing with UTC-aware now()
- [Majeur] ai_agent.py: fix 'raw' in dir() -> 'raw' in locals() so the
  JSON parse error log actually shows the raw response
- [Mineur] export print: use datetime.now(UTC) instead of datetime.now()

Frontend (React):
- [Majeur] consultation.tsx: show startsAt (not endsAt) for upcoming
  consultations; add startsAt variable
- [Majeur] consultations-list.tsx: same fix for the list view
- [Mineur] home.tsx: guard new Date(idea.createdAt) against null
- [Mineur] admin.tsx: check HTTP status in exportCsv XHR before creating
  download link; show error toast on non-200

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/voix-du-peuple/src/pages/admin.tsx

@@ -658,6 +658,10 @@ export default function Admin() {
     req.setRequestHeader("Authorization", `Bearer ${token}`);
     req.responseType = "blob";
     req.onload = () => {
+      if (req.status !== 200) {
+        toast({ title: "Erreur export", description: `Erreur ${req.status} lors de l'export CSV.`, variant: "destructive" });
+        return;
+      }
       const a = document.createElement("a");
       a.href = URL.createObjectURL(req.response);
       a.setAttribute("download", "contributions.csv");