perf(build): skip AI-author phase on deploy builds (#2761)

[jung-thomas]

Closes #2761 (quick wins #1 and #2 from the release retro; the mtar-size, 502-resilience, and deploy-entry-point consolidation items remain for separate PRs).

What

A deploy build renders ~1400 tutorial bodies that the approuter discards (tutorial HTML is served from HANA BLOBs, not static files — see CLAUDE.md). The expensive AI-author pass (AI-quiz expansion + json-render challenge specs) on that throwaway render is pure wall-clock waste, and when there's no AI Hub binding it burns up to AI_AUTHOR_BUILD_CAP (200) per-step throw-and-log cycles.

Two quick wins, behind one decision point:

1. SKIP_AI_AUTHOR flag (default off). build:deploy sets it via cross-env (existing repo pattern, cf. seed-ai-quizzes), skipping the AI phase on deploy builds. rebuild-content.yml leaves it unset, so HANA-served bodies — the ones that actually keep their quizzes — are unaffected.
2. Fail-open on missing AI Hub binding. The gate resolves the binding once per build; a missing deploymentId skips the whole phase with a single warning instead of 200 per-step throws.

How

• New pure, unit-tested module scripts/lib/ai-author-gate.ts → resolveAiAuthorGate() returns { enabled, reason }. Resolver is injected, so the decision is testable without a real binding.
• scripts/fetch-tutorials.ts resolves the gate once, logs [ai-author] ENABLED/DISABLED — <reason>, and gates both AI call sites. When disabled, unmaterialized [AUTOAUTHOR_*] placeholders are dropped so none leak into the page; hand-authored [VALIDATE_N] questions flow through untouched.
• package.json: build:deploy → cross-env SKIP_AI_AUTHOR=1 npm run build:all.

Testing

• test/lib/ai-author-gate.test.ts — 5 tests, all green: skip short-circuits before probing; enabled when binding resolves (probed exactly once); fail-open on throw and on null deploymentId; env-var read.
• Full unit suite: the 26 failing files are pre-existing missing-build-artifact / native-dep failures (missing core.bundle.mjs / parsers.bundle.mjs, mediapipe selfie, Vue islands needing npm run setup) — verified to fail identically on clean origin/DEV with fetch-tutorials.ts reverted. None import this change.

Not done here (follow-ups on #2761)

mtar size (#3), 502 upload resilience (#4), deploy-entry-point consolidation (#5).