CE: Add og:image + Twitter large-card social previews

[ericflo]

What

Adds a real 1200x630 branded social-share image so links to modelrelay.io render as professional cards on Twitter, Slack, Discord, iMessage, LinkedIn, etc.

Why

Verified gap before doing the work: `curl https://modelrelay.io/ | grep -iE "og:image|twitter:image"` returned nothing. Every page_shell page and the two static templates (index.html, pricing.html) declared og:title/description/url but no image, and twitter:card was set to `summary` (small thumbnail). Sharing a modelrelay.io link today shows a bare URL with no preview, which looks unfinished next to peers like Stripe, Vercel, Linear.

Changes

• New `crates/modelrelay-cloud/assets/og-image.png` — 1200x630 PNG, purple-to-blue gradient with the ModelRelay wordmark, tagline, and a minimalist relay icon. Generated with OpenAI gpt-image-1.5 at low quality, cropped and resized to exact social-card dimensions with Pillow.
• New `/og-image.png` route in `crates/modelrelay-cloud/src/routes/mod.rs`. The PNG is baked into the binary via `include_bytes!` so it ships everywhere the cloud service runs and the CSP `img-src 'self' data:` header stays unchanged (no external host needed). Served with `Cache-Control: public, max-age=86400, immutable`. `/og-image.png` is also added to `SESSION_EXEMPT_ROUTES` so the session guard does not block it.
• `page_shell_custom` in `crates/modelrelay-web/src/templates.rs` now emits `og:image`, `og:image:width`, `og:image:height`, `og:image:alt`, `twitter:image`, `twitter:image:alt`, and `twitter:card` upgraded from `summary` to `summary_large_image`.
• The two static templates `crates/modelrelay-cloud/templates/index.html` and `crates/modelrelay-cloud/templates/pricing.html` were updated the same way so every page has a full social-card set.

Tests

• New `og_image_returns_png` — hits `/og-image.png`, asserts 200, `Content-Type: image/png`, `Cache-Control` with a `max-age`, body > 1000 bytes, and leading PNG magic bytes.
• New `landing_page_includes_og_image_meta` — confirms the rendered `/` HTML contains og:image, twitter:image, and `twitter:card=summary_large_image`.
• Updated `pricing_page_has_og_url_and_canonical` to require `summary_large_image` instead of `summary`.
• `cargo nextest run -p modelrelay-cloud -p modelrelay-web` → 70 passed, 0 failed.
• `cargo fmt --check` and `cargo clippy -p modelrelay-cloud -p modelrelay-web --tests -- -D warnings` clean.

Manual verification after deploy

• `curl -sS https://modelrelay.io/ | grep -iE "og:image|twitter:image|twitter:card"` should show og:image + twitter:image + `summary_large_image`.
• `curl -sS -I https://modelrelay.io/og-image.png\` should return `image/png` + `public, max-age=86400`.
• Paste `https://modelrelay.io/\` into https://cards-dev.twitter.com/validator and https://www.opengraph.xyz/ to confirm the card renders.