[Demo] Add admin demo-mode toggle + generic website ingest source

- AppSetting: DemoMode, WebsitesEnabled, WebsiteUrls
- Facility.IsDemo flag; SeedData split into SeedReferenceAsync (always)
  + SeedDemoAsync/ClearDemoAsync (idempotent, toggleable at runtime)
- WebsiteListingSource: scrape any admin-configured URL (og:title + content)
- Admin Settings: seed/clear demo card, demo-mode checkbox, website source
  fields; Program.cs seeds demo when DemoMode on (or in Development)
- EF migration DemoModeAndWebsites

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

src/JobsMedical.Web/Models/AppSetting.cs

@@ -44,6 +44,13 @@ public class AppSetting
     public bool BaleEnabled { get; set; } = false;
     [MaxLength(200)] public string? BaleBotToken { get; set; }
 
+    /// <summary>Demo mode — keep the sample Tehran board seeded/visible (for showcasing).</summary>
+    public bool DemoMode { get; set; } = false;
+
+    public bool WebsitesEnabled { get; set; } = false;
+    /// <summary>Generic web pages to scrape, one URL per line.</summary>
+    [MaxLength(4000)] public string? WebsiteUrls { get; set; }
+
     public bool DivarEnabled { get; set; } = false;
     [MaxLength(60)] public string? DivarCity { get; set; } = "tehran";
     /// <summary>Divar search terms, one per line or comma-separated.</summary>