feat: Admin Operations SIEM — alerts, notifications, pre-built rules

- Add pluggable notification system (webhook, Slack, Teams) with retry
- Add alert deduplication: same rule + actor within 15 min = one alert
- Add 10 pre-built admin-ops rule templates seeded on startup:
  - Failed Conditional Access, After-Hours Admin Activity
  - New Application Registration, Admin Role Assignment
  - License Change, Bulk User Deletion
  - Device Compliance Failure, Exchange Transport Rule Change
  - Service Principal Credential Added, External Sharing Enabled
- Add /api/alerts, /api/alerts/{id}/status, /api/alerts/summary endpoints
- Add alert dashboard to frontend with status filters and ack/resolve buttons
- Add alert summary badge in hero header (high/medium/low counts)
- New env vars: ALERT_WEBHOOK_URL, ALERT_WEBHOOK_FORMAT, ALERT_DEDUPE_MINUTES

backend/frontend/style.css

@@ -691,6 +691,124 @@ input {
   font-size: 12px;
 }
 
+/* Alert summary in hero */
+.alert-summary {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  background: rgba(255, 255, 255, 0.04);
+  border: 1px solid var(--border);
+  border-radius: 999px;
+  padding: 6px 14px;
+}
+
+.alert-badge {
+  min-width: 22px;
+  height: 22px;
+  border-radius: 999px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 11px;
+  font-weight: 700;
+  color: #0b1220;
+}
+
+.alert-badge--high {
+  background: #ef4444;
+}
+
+.alert-badge--medium {
+  background: #f97316;
+}
+
+.alert-badge--low {
+  background: #3b82f6;
+}
+
+.alert-label {
+  font-size: 12px;
+  color: var(--muted);
+}
+
+.alert-open-count {
+  font-size: 13px;
+  color: var(--muted);
+}
+
+.alert-filters {
+  display: flex;
+  gap: 10px;
+  margin-bottom: 12px;
+}
+
+.alert-filters select {
+  padding: 8px 12px;
+  border-radius: 8px;
+  border: 1px solid var(--border);
+  background: rgba(255, 255, 255, 0.02);
+  color: var(--text);
+  font-size: 13px;
+}
+
+.alerts-list {
+  display: flex;
+  flex-direction: column;
+  gap: 10px;
+}
+
+.alert-card {
+  border: 1px solid var(--border);
+  border-radius: 12px;
+  padding: 12px 14px;
+  background: rgba(255, 255, 255, 0.02);
+  border-left: 3px solid transparent;
+}
+
+.alert-card--high {
+  border-left-color: #ef4444;
+}
+
+.alert-card--medium {
+  border-left-color: #f97316;
+}
+
+.alert-card--low {
+  border-left-color: #3b82f6;
+}
+
+.alert-card__meta {
+  display: flex;
+  gap: 8px;
+  align-items: center;
+  margin-bottom: 6px;
+  flex-wrap: wrap;
+}
+
+.alert-card__meta small {
+  color: var(--muted);
+  font-size: 12px;
+}
+
+.alert-card strong {
+  font-size: 14px;
+  display: block;
+  margin-bottom: 4px;
+}
+
+.alert-card p {
+  margin: 0 0 10px;
+  font-size: 13px;
+  color: var(--muted);
+  line-height: 1.45;
+}
+
+.alert-card__actions {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+}
+
 @media (max-width: 640px) {
   .topbar {
     flex-direction: column;