seo: add og:image, Twitter Cards, meta tags

index.html:
- Add og:image with dimensions (1200x630)
- Add Twitter Cards (summary_large_image)

terms.html, privacy.html, sla.html:
- Add meta description
- Add robots, canonical URL
- Add Open Graph tags

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

index.html

@@ -17,8 +17,18 @@
   <meta property="og:url" content="https://hosting.anvil.it.com">
   <meta property="og:title" content="Anvil Hosting - 엔지니어가 관리하는 Linux 서버">
   <meta property="og:description" content="엔지니어가 직접 튜닝한 Linux 서버. 고객의 문제가 해결될 때까지, 우리는 멈추지 않습니다.">
+  <meta property="og:image" content="https://hosting.anvil.it.com/og-image.png">
+  <meta property="og:image:width" content="1200">
+  <meta property="og:image:height" content="630">
+  <meta property="og:image:alt" content="Anvil Hosting - 엔지니어가 관리하는 Linux 서버">
   <meta property="og:locale" content="ko_KR">
 
+  <!-- Twitter Cards -->
+  <meta name="twitter:card" content="summary_large_image">
+  <meta name="twitter:title" content="Anvil Hosting - 엔지니어가 관리하는 Linux 서버">
+  <meta name="twitter:description" content="엔지니어가 직접 튜닝한 Linux 서버. 고객의 문제가 해결될 때까지, 우리는 멈추지 않습니다.">
+  <meta name="twitter:image" content="https://hosting.anvil.it.com/og-image.png">
+
   <!-- Favicon -->
   <link rel="icon" type="image/svg+xml" href="/favicon.svg">