feat(analytics): instance census, full capture, richer error context (#511)

Add a once-per-boot instance_started event (arch, os, deploy_mode,
gpu_present) so the fleet architecture mix is measurable. It reuses the
existing per-instance instance_id and is exempt from the volume sample
rate, since a census that fires once per boot must not be thinned.

Restore useful capture depth now that the sponsored plan removes the
quota pressure behind the earlier hardening:

- PostHog sample rate 0.1 to 1.0 (full analytics when enabled); the
  property allowlist still blocks file data.
- Sentry per-instance ceiling 20 to 500/hr, breadcrumb trail restored
  (sanitized: urls/paths redacted, data payloads dropped), full stack
  paths kept; local vars, request bodies, and PII still dropped. Both
  api and web.

Honor ANALYTICS_ENABLED=false as an opt-out alias: it was documented on
the Docker Hub README but never wired in 2.x, so anyone who set it was
still tracked.

All capture stays behind the analytics opt-out gate.

.env.example

@@ -86,6 +86,7 @@ LOG_DIR=./data/logs            # rotating log ring for support bundles
 # Runtime kill switch: set to 0 to disable ALL telemetry (Sentry + PostHog)
 # for this instance without rebuilding. The in-app admin toggle does the same
 # from Settings; this env var also covers boot-time crashes and CI fleets.
+# ANALYTICS_ENABLED=false is honored as an alias (0/off also work).
 # SNAPOTTER_TELEMETRY=1
 # Label this instance's error reports (shows as the Sentry environment).
 # SNAPOTTER_ENV=production