Add automatic retention purging for the Diagnostic and Audit logs

The diagnostic log already rings-buffer to 500 rows, but the audit
log had no cap at all and would grow forever. Adds an opt-in
age-based purge under Settings -> Logs: keep entries for N days,
checked on a configurable interval (hourly through monthly), plus a
manual "Purge now" button. Reuses the existing node-schedule-style
reschedule-on-settings-change pattern from the secret/Tailscale
expiry checkers, but as a plain setInterval since "how often" here is
an interval rather than a specific daily time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/App.tsx

@@ -23,6 +23,7 @@ import NotificationSettings from "./pages/settings/NotificationSettings";
 import BadgeSettings from "./pages/settings/BadgeSettings";
 import DisplaySettings from "./pages/settings/DisplaySettings";
 import CacheSettings from "./pages/settings/CacheSettings";
+import LogSettings from "./pages/settings/LogSettings";
 import AppShell from "./layout/AppShell";
 import { setDateTimeSettings } from "./utils/date";
 import { setPageSize } from "./utils/pageSize";
@@ -126,6 +127,7 @@ export default function App() {
           <Route path="badges" element={<BadgeSettings />} />
           <Route path="display" element={<DisplaySettings />} />
           <Route path="cache" element={<CacheSettings />} />
+          <Route path="logs" element={<LogSettings />} />
         </Route>
       </Routes>
     </AppShell>