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>

pubspec.lock

@@ -79,7 +79,7 @@ packages:
       path: "../fai_dart_sdk"
       relative: true
     source: path
-    version: "0.7.1"
+    version: "0.8.0"
   fake_async:
     dependency: transitive
     description: