fix: HTTPS URLs in sitemap, robots, canonical + og:image on homepage

- Add UseForwardedHeaders middleware so Request.Scheme = "https" behind nginx
- Add SITE_BASE_URL env var fallback for sitemap.xml, robots.txt, and all
  Razor page canonical/og URLs — set it to https://draletaha.ir in .env
- Add og:image to homepage using hero photo
- Add SITE_BASE_URL to docker-compose.yml environment block

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

DrSousan.Api/Pages/Blog/Index.cshtml

@@ -4,7 +4,7 @@
 @section Head {
     <title>@ViewData["Title"]</title>
     <meta name="description" content="مقالات تخصصی دکتر سوسن آل‌طه درباره زیبایی پوست، بوتاکس، فیلر، لیزر و مراقبت از پوست." />
-    <link rel="canonical" href="@(Request.Scheme + "://" + Request.Host + "/blog")" />
+    <link rel="canonical" href="@((Environment.GetEnvironmentVariable("SITE_BASE_URL")?.TrimEnd('/') ?? (Request.Scheme + "://" + Request.Host)) + "/blog")" />
     <style>
       /* ─── Blog Hero ─────────────────────────────────────────────── */
       .blog-hero{background:linear-gradient(135deg,var(--gold-pale) 0%,#EDE0CA 100%);padding:6rem 2rem 3rem;text-align:center}