Breadcrumbs: visible trail + BreadcrumbList JSON-LD

Add SeoJsonLd.Breadcrumb + Crumb record + _Breadcrumbs partial, and wire a trail
into the Jobs/Shifts list (landing) and detail pages: خانه › استخدام/شیفت › {نقش}
› {شهر|عنوان}. The role crumb links to the role landing page (more internal
links), and Google can show the breadcrumb path in results. Detail pages emit it
alongside the existing JobPosting JSON-LD.

Improvement 5 of the backlog (SEO).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/JobsMedical.Web/Services/SeoJsonLd.cs

@@ -125,9 +125,32 @@ public static class SeoJsonLd
         },
     }, Opts));
 
+    /// <summary>BreadcrumbList JSON-LD from an ordered crumb trail (relative URLs are made absolute).
+    /// Google can then show the breadcrumb path in search results.</summary>
+    public static string Breadcrumb(IReadOnlyList<Crumb> items, string baseUrl)
+    {
+        var els = new List<object>();
+        for (var i = 0; i < items.Count; i++)
+        {
+            var el = new Dictionary<string, object?> { ["type"] = "ListItem", ["position"] = i + 1, ["name"] = items[i].Name };
+            if (!string.IsNullOrEmpty(items[i].Url))
+                el["item"] = items[i].Url!.StartsWith("http") ? items[i].Url : baseUrl + items[i].Url;
+            els.Add(el);
+        }
+        return Fix(JsonSerializer.Serialize(new Dictionary<string, object?>
+        {
+            ["@context"] = "https://schema.org",
+            ["@type"] = "BreadcrumbList",
+            ["itemListElement"] = els,
+        }, Opts));
+    }
+
     // Nested anonymous objects use "type"/"queryyy" placeholders for @type / query-input;
     // restore the @-prefixed schema.org keys here.
     private static string Fix(string json) => json
         .Replace("\"type\":", "\"@type\":")
         .Replace("\"queryyy\":", "\"query-input\":");
 }
+
+/// <summary>One step in a breadcrumb trail. <see cref="Url"/> is null for the current (last) page.</summary>
+public record Crumb(string Name, string? Url = null);