fix: short-circuit logfire proxy when browser_observability is disabled (#1289)
the frontend toggle from #1288 only prevents new page loads from
calling initObservability(). stale cached clients (Cloudflare Pages)
continue hammering POST /logfire-proxy — 3,458 requests in 24 minutes
averaging 1.9s each, saturating the threadpool and causing /tracks/top
to take 10-18s.
guard the backend endpoint directly: return 204 immediately when the
flag is off, so no stale client can reach logfire_proxy().
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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