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>

index.html

@@ -5,6 +5,7 @@
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <meta name="description" content="HotelSync — современная PMS система для отелей" />
+    <meta name="robots" content="noindex, nofollow" />
     <title>HotelSync</title>
     <link rel="preconnect" href="https://fonts.googleapis.com" />
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />