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

locales/en.json

@@ -1092,6 +1092,9 @@
   "admin_settings_section_backups": "Backups",
   "admin_settings_section_devices": "Devices",
   "admin_settings_section_support": "Support",
+  "admin_settings_section_system": "System",
+  "admin_settings_field_telemetry_enabled_label": "Anonymous install analytics",
+  "admin_settings_field_telemetry_enabled_description": "Sends one anonymous heartbeat per day (version, OS, locale, user-count range). No personal data, tokens or domains. Helps gauge how many installs are active and which versions are in use. Toggling this off takes effect without a restart.",
   "admin_settings_subsection_common": "Common",
   "admin_settings_subsection_checkout": "Checkout",
   "admin_settings_subsection_remnawave": "Remnawave",

locales/ru.json

@@ -1092,6 +1092,9 @@
   "admin_settings_section_backups": "Бэкапы",
   "admin_settings_section_devices": "Устройства",
   "admin_settings_section_support": "Поддержка",
+  "admin_settings_section_system": "Система",
+  "admin_settings_field_telemetry_enabled_label": "Анонимная статистика установки",
+  "admin_settings_field_telemetry_enabled_description": "Раз в сутки отправляет обезличенный сигнал: версия, ОС, локаль и число пользователей в виде диапазона. Без персональных данных, токенов и доменов. Помогает оценить число активных установок и используемые версии. Отключение применяется без перезапуска.",
   "admin_settings_subsection_common": "Общие",
   "admin_settings_subsection_checkout": "Оформление оплаты",
   "admin_settings_subsection_remnawave": "Remnawave",