Admin-only diagnostic for observability wiring (PostHog + Sentry)

Report PostHog + Sentry wiring status for post-deploy verification.

Returns: {"posthog": {"initialized": bool, "host": str, "key_prefix": str | None, "last_capture_ts": str | None, "last_capture_event": str | None}, "sentry": {"dsn_set": bool}}

Authorization: requires the caller's email to appear in ADMIN_EMAILS. 401 for unauthenticated, 403 for authenticated non-admin.

The last_capture_* fields are process-local — they reflect the most recent successful enqueue in this worker. In a multi-worker prod pod, expect them to lag and vary between requests; that's fine for "is anything firing at all?" verification.