feat(admin): add local activity log + in-memory dismissal cache

Second piece of #419: a generic, local-only, append-only activity log
(datastore.RecordActivity/GetActivityRecords, one file per event under
stats/activity/<kind>/, same shape as SaveUsageStats) meant to back the
upcoming announcement-banner dismissals and be reusable for other admin-UI
action kinds later.

The read path never touches disk: a scoped startup scan folds prior
dismissals into an in-memory map once, RecordDismissal updates it
write-through. Same id can recur with a new timestamp (re-shown, dismissed
again) — it's a log, not a keyed store.

Not wired to anything user-facing yet — no announcements exist to dismiss.

Refs #419

data/.gitignore

@@ -5,5 +5,6 @@ default/
 dns/
 interactions/
 parity_mismatches/
+stats/
 patterns.json
 settings.json