feat: anonymous opt-out install telemetry beacon

Add a once-a-day anonymous heartbeat (PostHog) so maintainers can see
active installs and version/OS breakdowns. Self-hosted friendly: opt out
via TELEMETRY_ENABLED in .env or the Admin -> System toggle (applied
without a restart), or by clearing the endpoint/key.

- Share version resolution in bot/utils/app_version.py so the admin
  sidebar and the beacon report the same build version
- TelemetryWorker sends an opaque install id plus coarse facts only
  (version, OS/arch, python, locale, enabled providers, user-count
  range); never tokens, domains or user data
- Register the worker in main_worker.py behind a Redis single-flight lock
- Expose TELEMETRY_* settings and an Admin -> System manifest toggle
- Document the payload and opt-out in docs/configuration/telemetry.md
- Cover bucketing, payload shape and anonymity with tests

docs-site/astro.config.mjs

@@ -71,6 +71,7 @@ export default defineConfig({
           items: [
             { label: 'Переменные окружения', slug: 'configuration/env-vars' },
             { label: 'Безопасность', slug: 'configuration/security' },
+            { label: 'Телеметрия', slug: 'configuration/telemetry' },
           ],
         },
         {