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/settingsStore.ts

@@ -57,6 +57,14 @@ export interface DisplaySettings {
   pageSize: number;
 }
 
+export interface LogRetentionSettings {
+  enabled: boolean;
+  /** Diagnostic log and audit log entries older than this are deleted. */
+  retentionDays: number;
+  /** How often the purge job runs, in hours. */
+  intervalHours: number;
+}
+
 export interface AppSettings {
   gotify: GotifySettings;
   ntfy: NtfySettings;
@@ -66,6 +74,7 @@ export interface AppSettings {
   providerColors: ProviderColors;
   integrationColors: IntegrationColors;
   display: DisplaySettings;
+  logRetention: LogRetentionSettings;
 }
 
 const DEFAULTS: AppSettings = {
@@ -85,6 +94,7 @@ const DEFAULTS: AppSettings = {
   providerColors: {},
   integrationColors: {},
   display: { dateFormat: "ymd", timeFormat: "24h", pageSize: 20 },
+  logRetention: { enabled: false, retentionDays: 90, intervalHours: 24 },
 };
 
 const KEYS = Object.keys(DEFAULTS) as (keyof AppSettings)[];