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>

sla.html

@@ -4,6 +4,17 @@
   <meta charset="utf-8"/>
   <meta content="width=device-width, initial-scale=1.0" name="viewport"/>
   <title>서비스 수준 협약 (SLA) - Anvil Hosting</title>
+  <meta name="description" content="Anvil Hosting SLA (서비스 수준 협약). 99.9% 가용성 보장, 장애 대응 시간, 보상 정책을 안내합니다.">
+  <meta name="robots" content="index, follow">
+  <link rel="canonical" href="https://hosting.anvil.it.com/sla.html">
+
+  <!-- Open Graph -->
+  <meta property="og:type" content="website">
+  <meta property="og:url" content="https://hosting.anvil.it.com/sla.html">
+  <meta property="og:title" content="서비스 수준 협약 (SLA) - Anvil Hosting">
+  <meta property="og:description" content="Anvil Hosting SLA. 99.9% 가용성 보장 및 보상 정책을 확인하세요.">
+  <meta property="og:locale" content="ko_KR">
+
   <link rel="icon" type="image/svg+xml" href="/favicon.svg">
   <link href="https://fonts.googleapis.com" rel="preconnect"/>
   <link crossorigin="" href="https://fonts.gstatic.com" rel="preconnect"/>