feat: add opt-in anonymous telemetry reporting

Add an opt-in telemetry system that sends anonymised instance
statistics to the public API once per day, which proxies them to
PostHog when `POSTHOG_API_KEY` is configured.

- Add `packages/core/src/telemetry.ts` — `performTelemetryReport`
  and `isTelemetryEnabled`; user and title counts are bucketed
  before sending to avoid exposing exact figures
- Add `getInstanceId()` to `settings.ts` — generates and persists a
  stable UUIDv7 for the instance
- Schedule a daily `telemetryReport` cron job (00:30)
- Add `POST /v1/telemetry` to `apps/public-api`; forwards payload
  to PostHog or returns 204 silently if key is absent
- Add `admin.telemetry` and `admin.toggleTelemetry` oRPC procedures
  for inspecting and toggling the setting
- Expose `instanceId` on `system.publicInfo`

apps/server/src/orpc/procedures/system.ts

@@ -3,7 +3,11 @@ import {
   isOidcConfigured,
   isPasswordLoginDisabled,
 } from "@sofa/auth/config";
-import { getUserCount, isRegistrationOpen } from "@sofa/core/settings";
+import {
+  getInstanceId,
+  getUserCount,
+  isRegistrationOpen,
+} from "@sofa/core/settings";
 import { isTmdbConfigured } from "@sofa/tmdb/config";
 import { tmdbImageUrl } from "@sofa/tmdb/image";
 import { os } from "../context";
@@ -30,6 +34,7 @@ export const publicInfo = os.system.publicInfo.handler(async () => {
     .filter(Boolean) as string[];
 
   return {
+    instanceId: getInstanceId(),
     tmdbConfigured: isTmdbConfigured(),
     userCount: getUserCount(),
     registrationOpen: isRegistrationOpen(),