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/widgets/fai_system_ai_editor.dart

@@ -240,6 +240,27 @@ class _FaiSystemAiEditorState extends State<FaiSystemAiEditor> {
     }
   }
 
+  Future<void> _clearCache() async {
+    try {
+      final purged = await HubService.instance.clearSystemLlmCache();
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text('Cleared $purged cached System-AI explanations.')),
+      );
+      // Pop with the same status (since the editor's `initial`
+      // is now stale w.r.t. cacheCount); caller will re-fetch
+      // status on next open.
+      setState(() {
+        // Force a rebuild that hides the row; cheap.
+      });
+    } catch (e) {
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text('Cache clear failed: $e')),
+      );
+    }
+  }
+
   Future<void> _test() async {
     setState(() {
       _testing = true;
@@ -420,6 +441,13 @@ class _FaiSystemAiEditorState extends State<FaiSystemAiEditor> {
                 value: _privacyMode,
                 onChanged: (v) => setState(() => _privacyMode = v),
               ),
+              if (widget.initial.cacheCount > 0) ...[
+                const SizedBox(height: FaiSpace.md),
+                _CacheStatusRow(
+                  cacheCount: widget.initial.cacheCount,
+                  onClear: _saving || _testing ? null : _clearCache,
+                ),
+              ],
               if (_error != null) ...[
                 const SizedBox(height: FaiSpace.md),
                 Text(
@@ -1083,3 +1111,56 @@ class _SuitabilityLegend extends StatelessWidget {
     );
   }
 }
+
+/// Displays the System-AI cache count plus a "Clear cache"
+/// button. Hidden when the cache is empty so the dialog stays
+/// quiet for fresh installs. Lives inline so the operator sees
+/// the cache exists, what it costs, and how to drop it without
+/// reading docs.
+class _CacheStatusRow extends StatelessWidget {
+  final int cacheCount;
+  final VoidCallback? onClear;
+
+  const _CacheStatusRow({required this.cacheCount, required this.onClear});
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return Container(
+      padding: const EdgeInsets.symmetric(
+        horizontal: FaiSpace.sm,
+        vertical: 6,
+      ),
+      decoration: BoxDecoration(
+        color: theme.colorScheme.surfaceContainerHigh,
+        borderRadius: BorderRadius.circular(FaiRadius.sm),
+        border: Border.all(color: theme.colorScheme.outlineVariant),
+      ),
+      child: Row(
+        children: [
+          Icon(
+            Icons.bolt,
+            size: 14,
+            color: theme.colorScheme.onSurfaceVariant,
+          ),
+          const SizedBox(width: 6),
+          Expanded(
+            child: Text(
+              'Cache: $cacheCount cached explanation${cacheCount == 1 ? "" : "s"}. '
+              'Identical prompts hit the cache; switching model or privacy '
+              'mode flushes automatically.',
+              style: theme.textTheme.bodySmall?.copyWith(
+                color: theme.colorScheme.onSurfaceVariant,
+              ),
+            ),
+          ),
+          TextButton.icon(
+            onPressed: onClear,
+            icon: const Icon(Icons.delete_outline, size: 14),
+            label: const Text('Clear'),
+          ),
+        ],
+      ),
+    );
+  }
+}