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>

backend/migrations/002_extend.sql

@@ -44,8 +44,8 @@ ALTER TABLE rooms
   ADD COLUMN IF NOT EXISTS photos             TEXT[]   NOT NULL DEFAULT '{}';
 
 -- Переименуем старые поля для единообразия
-ALTER TABLE rooms RENAME COLUMN IF EXISTS capacity       TO max_guests;
-ALTER TABLE rooms RENAME COLUMN IF EXISTS price_per_night TO base_rate;
+ALTER TABLE rooms RENAME COLUMN capacity       TO max_guests;
+ALTER TABLE rooms RENAME COLUMN price_per_night TO base_rate;
 
 -- ── Guests (база гостей) ────────────────────────────────────────────────────
 CREATE TABLE IF NOT EXISTS guests (