Add SEO meta tags and structured data

Add proper SEO to my [FRAMEWORK, e.g. Next.js App Router] site so pages show up well in Google and look good when shared.

For every page, set:
- A unique title (around 55 to 60 characters) and meta description (around 150 to 160 characters).
- Open Graph tags (og:title, og:description, og:image, og:url, og:type) and Twitter card tags so links preview nicely.
- A canonical URL.

Site-wide:
1. Create a sitemap.xml that lists all real pages, and a robots.txt that allows crawling and points to the sitemap.
2. Add JSON-LD structured data: Organization for the site, and the right type per page (Article for blog posts, Product for products, FAQPage where I have a FAQ). Fill it from the real page data, not placeholders.

My site is [DESCRIBE, e.g. a SaaS landing site with a blog] and the domain is [DOMAIN].

Set this up in a reusable way so I define the title, description, and image once per page and everything else follows. Show me the shared metadata helper first, then apply it to [an example page]. Do not invent page content or fake review counts in the schema.