Add privacy notice and harden deployment tooling

[adewale]

Summary

Publish a globally linked privacy notice for runner submissions, operational logs, Invisible Turnstile, cookies, and optional browser CDNs, while making production deployment fail early unless it uses Node 22 and a fresh Python vendor sync.

Why

The production audit found no privacy notice despite session-scoped Invisible Turnstile. The first deployment attempt also exposed a tooling failure: Node 26 could not start Pywrangler's Pyodide interpreter, and a stale sync token allowed python_modules/ to remain empty until Cloudflare rejected the upload for missing FastAPI.

What changed

• Add /privacy, canonical/OG/JSON-LD metadata, sitemap coverage, global footer links, and a 1200×630 social card.
• Disclose Cloudflare request metadata, structured event fields, submitted-code processing, Turnstile and its clearance cookie, esm.sh requests, retention, and contact options.
• Add /privacy to deployment smoke checks and SEO/social-card gates.
• Require Node 22 for Pywrangler commands and force pywrangler sync --force before deployment.
• Document the deployment failure mode and update setup guidance.

Verification

• make verify — 224 tests, browser contracts, 109 examples, SEO/cache, quality, Ruff, and generated checks
• scripts/format_examples.py --check
• make verify-python-version VERSION=3.13
• npm audit --audit-level=high
• git diff --check
• Confirmed the new privacy-route tests failed before implementation
• Confirmed Node 26 fails the deploy preflight and Node 22 passes
• Social-card provenance and JPEG validation pass for 120 cards

Risk

The content route is read-only and uses existing page/layout primitives. Deployment changes intentionally reject unsupported local Node versions; CI already runs Node 22. The forced vendor sync costs additional deploy time but prevents dependency-free Worker uploads.