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/index.ts

@@ -23,11 +23,13 @@ import { integrationsRouter } from "./routes/integrations.js";
 import { settingsRouter } from "./routes/settings.js";
 import { initSecretExpiryScheduler } from "./services/secretExpiryScheduler.js";
 import { initTailscaleKeyExpiryScheduler } from "./services/tailscaleKeyExpiryScheduler.js";
+import { initLogRetentionScheduler } from "./services/logRetentionScheduler.js";
 
 warnIfAuthNotConfigured();
 await runMigrations();
 await initSecretExpiryScheduler();
 await initTailscaleKeyExpiryScheduler();
+await initLogRetentionScheduler();
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const webDist = join(__dirname, "..", "..", "web", "dist");