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/Post.cshtml

@@ -2,7 +2,7 @@
 @model DrSousan.Api.Pages.Blog.PostModel
 @{
     var post        = Model.Post!;
-    var baseUrl     = Request.Scheme + "://" + Request.Host;
+    var baseUrl     = Environment.GetEnvironmentVariable("SITE_BASE_URL")?.TrimEnd('/') ?? (Request.Scheme + "://" + Request.Host);
     var canonicalUrl = baseUrl + "/blog/" + post.Slug;
     var ogImage     = ViewData["OgImage"]?.ToString() ?? "";
     var articleType = ViewData["ArticleType"]?.ToString() ?? "MedicalWebPage";