feat: copy raw event and AI explain in modal

- Add POST /api/events/{id}/explain endpoint that fetches event + related events
  and asks the LLM for a plain-language explanation with security context
- Add 'Copy' button to raw event modal (uses navigator.clipboard)
- Add 'Explain' button to raw event modal (only when AI_FEATURES_ENABLED)
- Show explanation in modal with markdown rendering
- Add CSS for modal actions and explanation panel
- Add tests for explain endpoint (404, no LLM key, mocked LLM success)

backend/frontend/style.css

@@ -364,6 +364,22 @@ input {
   margin-bottom: 10px;
 }
 
+.modal__actions {
+  display: flex;
+  gap: 8px;
+  align-items: center;
+}
+
+.modal__explanation {
+  background: rgba(255, 255, 255, 0.03);
+  border: 1px solid var(--border);
+  border-radius: 10px;
+  padding: 12px;
+  margin-bottom: 10px;
+  font-size: 14px;
+  line-height: 1.6;
+}
+
 .modal pre {
   background: rgba(255, 255, 255, 0.02);
   color: var(--text);