feat(studio): cached/Regenerate UX in audit Explain (v0.16.0)

Audit drilldown's explanation panel now surfaces whether the
answer came from the cache:
- "cached" pill next to the SYSTEM AI label, tooltip shows
  the original generation date + hit count.
- Latency renders as "orig 47000 ms" so the operator sees
  what a live call would have cost.
- Refresh icon next to the latency triggers Regenerate —
  drops the entry, re-asks the model.

System-AI editor shows a cache-status row when the cache is
non-empty with a one-click Clear button. Hidden on fresh
installs to keep the dialog quiet.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/pages/audit.dart

@@ -401,13 +401,16 @@ class _EventDetailDialogState extends State<_EventDetailDialog> {
     }
   }
 
-  Future<void> _explain() async {
+  Future<void> _explain({bool forceFresh = false}) async {
     setState(() {
       _explaining = true;
       _explanation = null;
     });
     final prompt = _buildPrompt(event, _aiStatus?.privacyMode ?? 'off');
-    final result = await HubService.instance.askAi(prompt);
+    final result = await HubService.instance.askAi(
+      prompt,
+      forceFresh: forceFresh,
+    );
     if (!mounted) return;
     setState(() {
       _explaining = false;
@@ -536,6 +539,9 @@ class _EventDetailDialogState extends State<_EventDetailDialog> {
                   explaining: _explaining,
                   result: _explanation,
                   privacyMode: _aiStatus?.privacyMode ?? 'off',
+                  onRegenerate: _explaining
+                      ? null
+                      : () => _explain(forceFresh: true),
                 ),
               ],
             ],
@@ -626,11 +632,16 @@ class _ExplanationPanel extends StatelessWidget {
   final bool explaining;
   final AskAiResult? result;
   final String privacyMode;
+  /// Triggered by the "Regenerate" affordance — passes
+  /// `forceFresh: true` so the next askAi skips the cache and
+  /// hits the live provider. Null while [explaining] is true.
+  final VoidCallback? onRegenerate;
 
   const _ExplanationPanel({
     required this.explaining,
     required this.result,
     required this.privacyMode,
+    required this.onRegenerate,
   });
 
   @override
@@ -671,15 +682,42 @@ class _ExplanationPanel extends StatelessWidget {
                     ? FaiPillTone.warning
                     : FaiPillTone.neutral,
               ),
+              if (result != null && result!.isSuccess && result!.cached) ...[
+                const SizedBox(width: FaiSpace.xs),
+                Tooltip(
+                  message:
+                      'Served from the local cache. Originally generated '
+                      '${result!.cachedAt.isEmpty ? "at unknown time" : "on ${result!.cachedAt}"}'
+                      '${result!.cacheHits > 1 ? " · ${result!.cacheHits} hits" : ""}.',
+                  child: const FaiPill(
+                    label: 'cached',
+                    tone: FaiPillTone.success,
+                    icon: Icons.bolt,
+                  ),
+                ),
+              ],
               const Spacer(),
               if (result != null && result!.isSuccess && result!.latencyMs > 0)
                 Text(
-                  '${result!.latencyMs} ms',
+                  result!.cached
+                      ? 'orig ${result!.latencyMs} ms'
+                      : '${result!.latencyMs} ms',
                   style: FaiTheme.mono(
                     size: 10,
                     color: theme.colorScheme.onSurfaceVariant,
                   ),
                 ),
+              if (result != null && result!.isSuccess && onRegenerate != null) ...[
+                const SizedBox(width: FaiSpace.sm),
+                Tooltip(
+                  message: 'Regenerate — skip cache, ask the model again',
+                  child: IconButton(
+                    icon: const Icon(Icons.refresh, size: 14),
+                    visualDensity: VisualDensity.compact,
+                    onPressed: onRegenerate,
+                  ),
+                ),
+              ],
             ],
           ),
           const SizedBox(height: FaiSpace.sm),