Implement SEO, Sitemap, SSR/Prerendering & Structured Data for Course Search Indexing

## Description

Currently, when students search Google for queries like `cs101 materials iitg` or `MA101 coursehub`, CourseHub pages do not show up in search results. Shared links (WhatsApp, Discord, Telegram) also don't render rich previews. This issue tracks implementing end-to-end Search Engine Optimization (SEO) so all course pages are indexed, rank effectively on search engines, and preview correctly when shared.

## Key Requirements

### 0. Server-Side Rendering / Prerendering for Bot Requests
- Social crawlers (WhatsApp, Discord, Telegram, Twitter/X) **do not execute JavaScript** — they only read the raw HTML of the initial response. `react-helmet-async` alone will not produce visible OG tags for them, since it injects tags client-side after JS runs.
- Googlebot does execute JS but relies on a slower, less reliable two-wave indexing process for CSR content, especially on lower-authority sites.
- Implement SSR (or a build-time/on-request prerender fallback, e.g. `prerender.io` or a static-generation step) for `/browse/*` routes so bots receive fully-rendered HTML with title, meta, and OG tags already present in the initial response.
- This is a **prerequisite** for Requirements #2 and #4 to function correctly — do this first.

### 1. Dynamic Course Sitemap (`sitemap.xml`) & `robots.txt`
- Generate/serve a `sitemap.xml` listing all public course routes (e.g. `/browse/CS101`, `/browse/MA101`), including `lastmod` timestamps.
- Sitemap should regenerate automatically when courses/materials are added or updated (not a static file that goes stale).
- Add `robots.txt` allowing search crawlers to index public `/browse/*` routes, and explicitly disallowing any private/authenticated routes.
- Add Google Search Console verification meta tag support.
- Add `` on each course page to prevent duplicate-content issues if a course is reachable via more than one URL pattern.
- Ensure invalid course codes (e.g. `/browse/CS999`) return a real HTTP 404, not a 200 response with "not found" text (avoids soft-404 penalties).

### 2. Search-Optimized Metadata (`react-helmet-async` + SSR from #0)
- Dynamically update page titles with search keywords:
- **Browse Course Page:** `{Course Code} - {Course Name} Study Materials | CourseHub IIT Guwahati`
- **Example:** `CS101 - Introduction to Computing Study Materials | CourseHub IIT Guwahati`
- Set `` targeting key terms: `IIT Guwahati`, `course materials`, `notes`, `PYQs`, `syllabus`.
- Clarify which content is publicly indexable vs. behind login — if materials require auth, only the course shell/summary should be indexed, not gated content.

### 3. Structured Data (Schema.org `Course` JSON-LD)
- Inject JSON-LD schema on course pages so search engines identify the content as an educational course:
- `provider`: `"IIT Guwahati"`
- `courseCode`: `"CS101"`
- `name`: `"Introduction to Computing"`

### 4. Social Sharing Previews (Open Graph & Twitter Cards)
- Add `og:title`, `og:description`, `og:image`, and `og:url` tags (served via SSR, see #0) so sharing links on WhatsApp, Discord, or Telegram displays rich card previews with course details.

## Acceptance Criteria
- [ ] `/browse/*` pages return fully-rendered HTML (title/meta/OG tags present) on first response, verified via `curl` or "View Source" — not just in the rendered DOM.
- [ ] `sitemap.xml` validates in Google Search Console with 0 errors and reflects newly added courses without manual intervention.
- [ ] Invalid course routes return HTTP 404.
- [ ] Each course page has a unique, keyword-targeted title and meta description.
- [ ] Google's Rich Results Test passes for the `Course` schema on a sample page.
- [ ] Facebook Sharing Debugger and Twitter Card Validator show correct title, description, and image for a sample course link.
- [ ] Lighthouse SEO score ≥ 90 on a sample course page.

## Out of Scope / Follow-up
- General Core Web Vitals / page speed optimization (affects ranking but is a separate effort).
- Content strategy (blog posts, backlinks, etc.) for off-page SEO.