Cold auth OAuth-callback latency (signup / create-project specs, ~30-90s)

#1601 fixed the cold-slot create saga (create-project.spec now passes on fresh slots), but the auth OAuth callback path is still slow on freshly deployed slots: specs/signup.spec.ts ("can sign up with an email one-time passcode") failed all 3 attempts at ~33s each on a fresh preview slot (2026-07-02, PR #1589 validation) — the browser parked on /api/iterate-auth/callback past its budget. tasks/os-cold-create-latency.md flagged this as the "same family, second data point": the callback (token exchange / auth DO / os-side fetch) is cold on first touch.

This is the last thing making preview e2e red on a truly-cold slot; the vitest engine lane is green. Fix it similarly to #1601 (heal/warm the cold path, batch/parallelize, or restore a sane budget) so signup + first-org onboarding land within the spec timeout on fresh deploys.

Related: [[project_preview_e2e_speedup]], tasks/os-cold-create-latency.md, tasks/raise-e2e-maxconcurrency.md (the auth callback + create latency are what kept e2e concurrency pinned low).