feat: Generate per-course metadata in courses/[courseId] instead of s…

[OkeQueen]

Generate per-course metadata in courses/[courseId]
Summary

Fixes #926

generateMetadata in courses/[courseId]/page.tsx was missing a per-course Open Graph image and canonical URL (title/description were already dynamic from an earlier commit).
Adds alternates.canonical and openGraph.url pointing to /courses/${courseId}, resolved against the existing metadataBase in src/app/layout.tsx.
Adds openGraph.images and twitter.images sourced from course.thumbnailUrl, omitted when a course has no thumbnail.
Changes
src/app/courses/[courseId]/page.tsx

return {
title: ${course.title} | TeachLink,
description: course.description,

• alternates: {

•  canonical: `/courses/${courseId}`,
  

• },
  openGraph: {
  title: ${course.title} | TeachLink,
  description: course.description,
  type: 'website',
  siteName: 'TeachLink',

•  url: `/courses/${courseId}`,
  

•  images: course.thumbnailUrl
  

•    ? [{ url: course.thumbnailUrl, alt: course.title }]
  

•    : undefined,
  

  },
  twitter: {
  card: 'summary_large_image',
  site: '@TeachLink',
  title: ${course.title} | TeachLink,
  description: course.description,

•  images: course.thumbnailUrl ? [course.thumbnailUrl] : undefined,
  

  },
  };
  Test plan
  npm install then npx tsc --noEmit (not run here — node_modules not installed in this environment)
  npm run dev, visit /courses/ for a course with and without thumbnailUrl, inspect for og:image, og:url, and
  Confirm Course Not Found case still returns the generic fallback metadata

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