Open Graph images

You are adding generated OG images to a Prelo project. Requires sharp (already a CMS dependency) and a `SITE_URL`.

1. **Template.** Create `og-template.js` exporting `renderSvg({ siteName, title, accent })` → an SVG string, 1200×630:
   - Background: near-white or the project's paper color; a 12px accent bar or corner mark in the project's brand color (read it from existing design tokens/config — do not invent one).
   - Site name small at top in the project's font stack (system-safe fallbacks only — SVG rasterization won't load webfonts unless embedded; embed the font as base64 `@font-face` in the SVG only if the project has a local font file).
   - Title large (64–88px), wrapped manually: break into lines of ~20 chars at word boundaries, max 4 lines, ellipsize beyond. Escape XML entities.

2. **Route.** Add to `cms.config.js` `routes:` (custom routes mount under `/api/x/`):

   ```js
   "GET /og/:type/:slug": async (req, res, db) => { /* … */ }
   ```

   (Custom routes are Express routes, so `:type`/`:slug` params work — see `prelo docs api`.) The handler must:
   - Look up the entry (published only) by type + slug; 404 if missing.
   - Compute a cache key `sha256(type + slug + title + updated_at)`; if `data/x-og-cache/{key}.png` exists, stream it.
   - Otherwise `sharp(Buffer.from(svg)).png()` → write to cache, then respond `image/png` with `Cache-Control: public, max-age=31536000, immutable`.
   - Create the cache dir in `hooks.boot`.

3. **Head wiring.** In the frontend's `seoTags`/head helper: `og:image` = the entry's `seo.ogImage` if set, else `{CMS_URL}/api/x/og/{type}/{slug}`, absolute. Add `og:image:width`/`height` (1200/630) and `twitter:card` = `summary_large_image`.

4. **Homepage.** Give the homepage a generated image too (site name + tagline), same route with a reserved slug or a dedicated `"GET /og-home"` route.

5. **Verify.** Fetch the route for one post — confirm a valid PNG, correct dimensions, second fetch served from cache (log or timing). Paste a URL into a share-preview debugger locally (e.g. `curl -s | head` for the meta tags) to confirm og:image resolves absolutely.