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

.env.example

@@ -73,3 +73,15 @@ FRONTEND_PORT=8082
 # Reverse proxy IPs/CIDRs trusted for X-Forwarded-For.
 # Keep loopback for local proxy; add your proxy network if needed.
 TRUSTED_PROXIES=127.0.0.1,::1
+
+# ─── Anonymous install telemetry (opt-out) ──────────────────────────────
+# Once a day the worker sends a single anonymous "heartbeat" so the project
+# maintainer can see how many installs are active and which versions/OSes are
+# used. It contains an opaque random install id and coarse facts only:
+# version, OS/arch, Python version, language, enabled payment providers and a
+# user-count RANGE (e.g. "51-200"). No bot token, domain, user data or any
+# personal information is ever sent. Full details: docs/configuration/telemetry.md
+#
+# Set to False to disable, or toggle it any time in Admin -> System ->
+# "Anonymous install analytics" (applies without a restart).
+TELEMETRY_ENABLED=True