feat: add anonymous instance telemetry

Adds a once-per-day heartbeat that lets the project see how many
instances run Bulwark, on what platforms, with what features enabled,
and roughly how many accounts they have. No email addresses, hostnames,
IPs, or any end-user data are ever sent.

- lib/telemetry: state file, payload builder, jittered scheduler,
  instance_id persistence at <data-dir>/.telemetry-id (delete to reset)
- app/api/admin/telemetry: admin API for status / set-consent /
  set-endpoint / send-now (all audit-logged)
- app/admin/telemetry: settings page with status, JSON payload preview,
  endpoint editor, send-now button, link to the privacy page
- instrumentation.node.ts: starts the scheduler on boot

Default state is enabled. The first heartbeat fires 1 hour after boot
so an admin who installs and immediately disables produces zero pings.
Disable via the settings UI, BULWARK_TELEMETRY=off (or
BULWARK_TELEMETRY_DISABLED=1), or by clearing the endpoint.

Account counts are bucketed (1, 2-5, 6-10, 11-50, 51-200, 201+) so a
small instance can't be re-identified by exact size. The /.telemetry-id
file can be deleted to mint a fresh instance_id.

Receiving collector is open source at bulwarkmail/dashboard. Self-host
your own and point at it via BULWARK_TELEMETRY_URL. Full schema,
retention (90d raw → aggregates), and lawful basis are documented at
bulwarkmail.org/docs/legal/privacy/telemetry.

app/api/admin/oauth/setup/route.ts

@@ -139,7 +139,7 @@ export async function POST(request: NextRequest) {
     const queryEntry = queryRes.methodResponses?.[0];
     if (!queryEntry || queryEntry[0] === 'error') {
       return NextResponse.json({
-        error: 'Stalwart denied OAuthClient/query — your Stalwart account likely lacks admin permissions.',
+        error: 'Stalwart denied OAuthClient/query - your Stalwart account likely lacks admin permissions.',
         detail: queryEntry?.[1],
       }, { status: 403 });
     }
@@ -187,7 +187,7 @@ export async function POST(request: NextRequest) {
     const setEntry = setRes.methodResponses?.[0];
     if (!setEntry || setEntry[0] === 'error') {
       return NextResponse.json({
-        error: 'Stalwart denied OAuthClient/set — admin permissions required.',
+        error: 'Stalwart denied OAuthClient/set - admin permissions required.',
         detail: setEntry?.[1],
       }, { status: 403 });
     }