cache bundled functions across deploys

[michaelfward]

two deploy-time fixes that compound:

1. cache bundled functions across deploys. actions/cache restores .netlify/functions/, deploy-ci re-stamps the manifest (netlify-cli only trusts it for 2 minutes), and netlify deploy reuses the zips. on a miss, netlify build produces the zips + manifest once (no double bundling) and the cache persists them.

2. bundle uploadthing instead of shipping its node_modules closure. the global external_node_modules = ["uploadthing"] made zisi ship uploadthing's entire dependency closure (the effect ecosystem, ~4900 files, 16-24MB) into every one of the 130 function zips - ~2GB per deploy, minutes of hashing + upload. esbuild bundles + treeshakes it fine today (the april commit that externalized it was a combined shotgun fix with the esbuild switch, never separately needed). zips drop ~60%+.

changes

file	change
deploy.yml / deploy-preview.yml	actions/cache step for .netlify/functions, exact-match key over every bundling input
scripts/deploy-ci.ts	hit: vite build only; miss: netlify build (bundles + writes manifest, falls back to plain vite build on failure); manifest re-stamped right before deploy
netlify.toml	drop external_node_modules = ["uploadthing"]

the cache key covers netlify/, scripts/ (guidance.ts inlines scripts/seed-guidance.ts), netlify.toml, lockfile, tsconfig. zisi output is deterministic (verified: bundling the same function twice gives identical sha256), so an exact-key restore is byte-identical to a fresh build. no restore-keys fallback on purpose - a near-miss restore would ship stale function code.

measured on this PR's previews

	before (prod baseline)	cache miss + slim zips	cache hit + slim zips
bundle 130 functions	~234s (inside deploy)	56.5s (netlify build)	skipped (22.9s vite only)
hashing	~4min	0.8s	~1s
function upload	~3min	~2.9min	~2.8min (per-file api overhead, not bytes - and netlify never dedupes function uploads even for byte-identical zips)
whole job	~14min	5m58s	5m20s

test plan

• preview run 1: cache miss, netlify build bundles, deploy logs "Deploying functions from cache"
• rerun of the same preview: cache hit restored, vite-only build, deploy reused the cached zips
• slim-zip miss run: bundling 187.9s -> 56.5s, hashing 4min -> 0.8s, job 5m58s
• slim-zip hit run: job 5m20s end to end
• preview functions respond: /api/guidance 401 (auth gate), /api/projects 200 with well-formed json (same as prod) - uploadthing module graph loads inlined
• post-merge: watch the first real image-upload path (thumbnails / og images) since the preview smoke doesn't exercise an actual uploadthing call

[github-actions]

🔎 Loop QA preview (all-k8s backend): https://pr-116--loop-qa-j63k5x.netlify.app