feat(proxy-logs): record per-attempt upstream timing on proxy log rows

[maxmad64bis]

⚠️ base-red inherited: #14963

Summary

Long requests could not separate upstream queue wait from generation time: no stored row said when the response headers or the first useful chunk arrived. Each attempt row now records both durations from send start (headers_ms, first_chunk_ms, nullable, migration 194), measured on the raw upstream body before any translation, and the proxy log detail shows them as "Headers after" and "First chunk after" (a dash when unknown, never 0). The queue/generation split is readable from the row and on screen without replaying logs.

Related Issues

• No linked issue — proactive observability improvement with no prior report.

Validation

• Change type: DB
• Focused tests and category gates from the golden path — 8/8 touched suites green; all nine new timing cases fail without the change
• npm run lint — ESLint on the touched files is clean
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/proxy-logs-attempt-timing.test.ts (new, 9 cases): column defaults and deferred update, sanitization, bounded pending registry (settle and flood), lazy body envelope (no stamp before the downstream read, status/headers kept, null settle on empty, failed or missing bodies).
• tests/unit/proxy-logger-observed-fields.test.tsx (1 new case): the detail view shows both durations, and a dash when they are unknown.

Coverage Notes

• open-sse/utils/upstreamStatusCapture.ts (capture wrapper): proxy-logs-attempt-timing.test.ts (lazy body envelope, settle on empty or failed bodies).
• src/lib/db/ (migration 194, columns, deferred update), src/lib/proxyLogger.ts and src/sse/handlers/proxyJournal.ts: the same file (column defaults, deferred update, sanitization, bounded pending registry).
• src/shared/components/ProxyLogDetail.tsx: proxy-logger-observed-fields.test.tsx.

Reviewer Notes

• The docs gate reports the migration count moving from 190 to 191 in README.md, AGENTS.md and llm.txt. Those counts are left untouched on purpose: per the maintainer's note on fix(proxies): steer selector to members without recent refusals #14804 they only move on the release train.
• One timing sanitizer is shared by the capture wrapper, the journal and the deferred patch; the stale no-unused-vars suppression for ProxyLogDetail.tsx (already unused at the base) is pruned so ESLint stays clean on that file.
• Inherited from the base (🔴 Release branch not green: release/v3.8.51 #14963): mutation-test-coverage --strict fails the same way on the release base; no file of this PR is involved.

Maintainer rework (merge-batch 2026-09-28)

• The first-byte body envelope and the deferred first_chunk_ms row patch are now opt-in via PROXY_LOG_FIRST_CHUNK_TIMING (default off, documented in .env.example and docs/reference/ENVIRONMENT.md). With it off the upstream Response passes through untouched (same object) and only headers_ms is recorded; the journal registers no pending link.
• The enveloped Response now keeps url, redirected and type from the upstream response (same approach as tlsClient.ts).
• Removed the duplicated sanitizeAttemptTiming; the journal reuses sanitizeTimingMs.
• Removed ensureProxyLogsColumns from the per-flush path (it already runs at init).
• New tests: identity preserved through the envelope; flag off (unset/false/0) returns the same Response unwrapped; flag off registers no deferred patch in the journal (control with flag on). They fail on the previous head (3/12) and pass now (12/12).
• Migration 194 is still the next free number on the tip.