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`

packages/core/package.json

@@ -16,6 +16,7 @@
     "./providers": "./src/providers.ts",
     "./settings": "./src/settings.ts",
     "./system-health": "./src/system-health.ts",
+    "./telemetry": "./src/telemetry.ts",
     "./tracking": "./src/tracking.ts",
     "./update-check": "./src/update-check.ts",
     "./webhooks": "./src/webhooks.ts"