Add docs, robots.txt and fix migration syntax

- backend/README.md: full API and DB documentation
- public/robots.txt: block all pages except root for SEO
- index.html: add noindex meta tag for inner SPA pages
- 002_extend.sql: fix RENAME COLUMN syntax (remove unsupported IF EXISTS)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/robots.txt

@@ -0,0 +1,5 @@
+User-agent: *
+Allow: /$
+Disallow: /
+
+Sitemap: https://app.hotelsync.ru/sitemap.xml