CE: Add /terms and /privacy legal pages

[ericflo]

What

Adds public legal pages at /terms and /privacy, wires them into the session-exempt list and sitemap, and links to both from the signup form.

Why

Stripe-gated SaaS products are expected to link Terms of Service and a Privacy Policy from checkout/signup, and users creating accounts at /signup were previously giving us their email with no privacy commitment on the page. Both paths previously 404'd on modelrelay.io.

Changes

• crates/modelrelay-cloud/src/routes/legal.rs (new): terms_page and privacy_page handlers rendered through modelrelay_web::templates::page_shell with per-page canonical/og:url for /terms and /privacy. Bodies cover the sections requested (acceptance, description of service, account responsibilities, acceptable use, payment/cancellation via Stripe, availability + rate limits, suspension/termination, disclaimers, limitation of liability, governing law CA/SF, changes, contact; for privacy: sign-up data, usage metadata, Stripe, cookies, API keys, use, sharing, retention, security, user rights, children, jurisdiction, changes, contact).
• crates/modelrelay-cloud/src/routes/mod.rs: registers pub mod legal;, adds /terms + /privacy routes, adds both to SESSION_EXEMPT_ROUTES, and adds <url><loc> entries to sitemap_xml.
• crates/modelrelay-cloud/src/routes/auth.rs: adds a small "By creating an account, you agree to our Terms of Service and Privacy Policy" line under the Create Account button in signup_form_html.
• crates/modelrelay-cloud/tests/http_routes.rs: four new tests covering 200 + <h1> + canonical/og:url on both pages, sitemap inclusion, and signup-form link presence. Also extends the existing sitemap test to assert /terms and /privacy are present.

Validation

• cargo fmt --all -- --check clean
• cargo clippy -p modelrelay-cloud --all-targets --no-deps -- -D warnings clean
• cargo test -p modelrelay-cloud — 30 passed, 0 failed (including the 4 new tests)

Post-deploy verification

After Deploy Cloud succeeds, expect:

• curl -s -o /dev/null -w "%{http_code}" https://modelrelay.io/terms → 200
• curl -s -o /dev/null -w "%{http_code}" https://modelrelay.io/privacy → 200
• Sitemap includes /terms and /privacy
• Signup form displays "By creating an account, you agree to..." with the two links