feat: deep SEO optimization — metadata, OG tags, sitemap, structured data, GA4

- Add metadataBase and localized generateMetadata to all pages (EN/FR)
- Add Open Graph and Twitter card defaults with branded OG image
- Add canonical URLs and hreflang alternates on every page
- Create robots.ts (allow all, block /admin/ and /api/)
- Create sitemap.ts with all routes x 2 locales and hreflang
- Add Organization, WebSite, and CreativeWork JSON-LD structured data
- Add GA4 (G-LD348WF8EM) with Consent Mode v2 (default denied for EEA)
- Add llms.txt for AI discoverability
- Add production nginx config for letsbe.biz

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.env.example

@@ -24,3 +24,6 @@ NEXT_PUBLIC_CALCOM_URL=https://cal.letsbe.biz
 
 # ── Site URL ──
 NEXT_PUBLIC_SITE_URL=https://staging.letsbe.biz
+
+# ── Google Analytics ──
+NEXT_PUBLIC_GA_ID=G-XXXXXXXXXX