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>

server/src/services/logRetentionScheduler.ts

@@ -0,0 +1,49 @@
+import { getSettings, getInternalFlag, setInternalFlag } from "./settingsStore.js";
+import { purgeOldLogs } from "./logRetention.js";
+
+const LAST_RUN_FLAG = "logRetentionLastRunAt";
+
+async function runPurge(): Promise<void> {
+  const { logRetention } = await getSettings();
+  if (!logRetention.enabled) return;
+  const result = await purgeOldLogs(logRetention.retentionDays);
+  await setInternalFlag(LAST_RUN_FLAG, new Date().toISOString());
+  if (result.diagDeleted || result.auditDeleted) {
+    console.log(
+      `[logRetention] purged ${result.diagDeleted} diagnostic log and ${result.auditDeleted} audit log entries older than ${logRetention.retentionDays} days`,
+    );
+  }
+}
+
+let currentTimer: ReturnType<typeof setInterval> | null = null;
+
+/** (Re)arms the periodic purge timer per the current settings. Call again after settings change. */
+export async function scheduleLogRetentionPurge(): Promise<void> {
+  if (currentTimer) {
+    clearInterval(currentTimer);
+    currentTimer = null;
+  }
+  const { logRetention } = await getSettings();
+  if (!logRetention.enabled) {
+    console.log("[logRetention] automatic purge disabled");
+    return;
+  }
+  const intervalMs = logRetention.intervalHours * 60 * 60 * 1000;
+  currentTimer = setInterval(() => {
+    runPurge().catch((err) => console.error("[logRetention] purge failed:", err));
+  }, intervalMs);
+  console.log(`[logRetention] automatic purge scheduled every ${logRetention.intervalHours}h, keeping ${logRetention.retentionDays} days`);
+}
+
+/** Runs immediately at startup if a purge is overdue (e.g. the server was down past the interval), then arms the periodic timer. */
+export async function initLogRetentionScheduler(): Promise<void> {
+  const { logRetention } = await getSettings();
+  if (logRetention.enabled) {
+    const lastRun = await getInternalFlag(LAST_RUN_FLAG);
+    const dueAt = lastRun ? new Date(lastRun).getTime() + logRetention.intervalHours * 60 * 60 * 1000 : 0;
+    if (Date.now() >= dueAt) {
+      await runPurge().catch((err) => console.error("[logRetention] purge failed:", err));
+    }
+  }
+  await scheduleLogRetentionPurge();
+}