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

backend/main_worker.py

@@ -27,6 +27,7 @@ from bot.services.backup_worker import BackupWorker
 from bot.services.locale_override_service import load_locale_overrides
 from bot.services.subscription_notification_worker import SubscriptionNotificationWorker
 from bot.services.tariff_worker import TariffTrafficWorker
+from bot.services.telemetry_worker import TelemetryWorker
 from bot.utils.message_queue import init_queue_manager
 from config.settings import get_settings
 
@@ -209,6 +210,8 @@ async def main() -> None:
     )
     backup_worker = BackupWorker(settings, bot, session_factory=session_factory)
     tasks.append(asyncio.create_task(backup_worker.run(), name="BackupWorker"))
+    telemetry_worker = TelemetryWorker(settings, session_factory)
+    tasks.append(asyncio.create_task(telemetry_worker.run(), name="TelemetryWorker"))
     tasks.append(asyncio.create_task(_panel_sync_loop(settings, session_factory, i18n, services)))
     for idx in range(max(1, settings.WEBHOOK_QUEUE_CONCURRENCY)):
         tasks.append(