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/Index.cshtml

@@ -1,6 +1,7 @@
 @page
 @model IndexModel
 @{
+    var siteBaseUrl = Environment.GetEnvironmentVariable("SITE_BASE_URL")?.TrimEnd('/') ?? (siteBaseUrl);
     var h = Model.Hero;
     var a = Model.About;
     var c = Model.Contact;
@@ -24,14 +25,21 @@
     <meta property="og:title"       content="@ViewData["Title"]" />
     <meta property="og:description" content="@h.GetValueOrDefault("subtitle","")" />
     <meta property="og:locale"      content="fa_IR" />
-    <link rel="canonical"           href="@(Request.Scheme + "://" + Request.Host + "/")" />
+    @if (!string.IsNullOrEmpty(heroImg))
+    {
+        var absHeroImg = heroImg.StartsWith("http") ? heroImg : (siteBaseUrl + heroImg);
+        <meta property="og:image"        content="@absHeroImg" />
+        <meta property="og:image:width"  content="1200" />
+        <meta property="og:image:height" content="630" />
+    }
+    <link rel="canonical"           href="@(siteBaseUrl + "/")" />
     <script type="application/ld+json">
     {
       "@@context":"https://schema.org",
       "@@type":["MedicalBusiness","LocalBusiness"],
       "name":"@siteName",
       "description":"@h.GetValueOrDefault("subtitle","")",
-      "url":"@(Request.Scheme + "://" + Request.Host)",
+      "url":"@(siteBaseUrl)",
       "telephone":"@c.GetValueOrDefault("phone","")",
       "address":{"@@type":"PostalAddress","addressLocality":"تهران","addressCountry":"IR","streetAddress":"@c.GetValueOrDefault("address","")"},
       "openingHours":"@c.GetValueOrDefault("hours","")",