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.

apps/api/src/lib/analytics-allowlist.ts

@@ -12,6 +12,7 @@ const ALLOWED: Record<string, ReadonlySet<string>> = {
     "status",
   ]),
   ai_bundle_action: new Set(["bundle_id", "action", "duration_ms"]),
+  instance_started: new Set(["arch", "os_platform", "deploy_mode", "gpu_present"]),
 };
 
 function isAllowedValue(value: unknown): boolean {