fix(org): favicon fallback, OG logo styling, and Discord embed CDN URLs (#714)

* fix(org): serve OG images from CDN for Discord embeds

Discord times out on slow dynamic OG generation (~3-5s). Persist generated
images to R2, point meta tags at the media CDN, and warm the API endpoint
on org-site page loads so crawlers get a fast static PNG URL.

Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

* docs: add org-site Open Graph and favicon guides

Document user-facing sharing behavior and contributor architecture
for dynamic OG images, favicon fallback, R2/CDN caching, and embed
cache invalidation.

Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

* fix(org-site): address PR review on OG CDN fallback and docs

- Fall back to API OG URL when CDN object is not yet available (HEAD check)
- Cancel warm-fetch response body to avoid connection pool exhaustion
- Align tenant-router OG edge cache TTL with one-hour API revalidation
- Fix favicon docs navigation and add Custom Domain tab screenshot
- Replace Chiberry Artistry example with AI Academy

Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

* fix(org-site): accept docker hostnames for OG warm URL

Use isValidHttpUrl for PRIVATE_SERVER_URL so internal values like
http://api:3081 work for SSR warm fetches in Docker Compose.

Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

apps/api/src/services/org/og.ts

@@ -7,6 +7,7 @@ import { getActiveOrganizationPlan, getOrganizationBySiteName } from '@cio/db/qu
 import { extractOrgTagline, renderOrgSiteOg } from '@api/utils/org-site-og';
 import { isOrgOnFreePlan } from '@cio/utils/plans';
 import { env } from '@cio/core/config/env';
+import { getCachedOrgSiteOgBuffer, getOrgSiteOgPublicUrl, persistOrgSiteOgImage } from '@api/services/org/og-cache';
 
 const DEFAULT_OG_IMAGE = 'https://brand.cdn.clsrio.com/og/classroomio-opengraph.jpg';
 const OG_VIEWPORT = { width: 1200, height: 630, deviceScaleFactor: 2 } as const;
@@ -20,6 +21,8 @@ export function getOrgSiteOgFallbackImageUrl(): string {
   return DEFAULT_OG_IMAGE;
 }
 
+export { getOrgSiteOgPublicUrl };
+
 function buildOrgSiteOgEtag(payload: {
   siteName: string;
   orgName: string;
@@ -59,8 +62,15 @@ export async function generateOrgSiteOgImage(siteName: string): Promise<{ buffer
     ...renderInput
   });
 
+  const cachedBuffer = await getCachedOrgSiteOgBuffer(siteName, etag);
+  if (cachedBuffer) {
+    return { buffer: cachedBuffer, etag };
+  }
+
   const { html, styles } = renderOrgSiteOg(renderInput);
   const buffer = await getCloudflarePngBuffer(html, styles, OG_VIEWPORT);
 
+  await persistOrgSiteOgImage(siteName, buffer, etag);
+
   return { buffer, etag };
 }