seo: add sitemap, robots.txt and optimised page titles/descriptions

- @astrojs/sitemap generates sitemap-index.xml at build time
- robots.txt points crawlers to the sitemap
- generators.ts extended with seoTitle and seoDescription per generator
- All generator pages now use seoTitle/seoDescription for <title> and <meta description>

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

astro.config.mjs

@@ -1,7 +1,10 @@
 import { defineConfig } from 'astro/config';
 import tailwindcss from '@tailwindcss/vite';
+import sitemap from '@astrojs/sitemap';
 
 export default defineConfig({
+  site: 'https://randify.pro',
+  integrations: [sitemap()],
   vite: {
     plugins: [tailwindcss()],
   },