[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/Program.cs

@@ -50,6 +50,8 @@ builder.Services.AddSingleton<JobsMedical.Web.Services.Scraping.IListingSource,
     JobsMedical.Web.Services.Scraping.DivarListingSource>();
 builder.Services.AddSingleton<JobsMedical.Web.Services.Scraping.IListingSource,
     JobsMedical.Web.Services.Scraping.MedjobsListingSource>();
+builder.Services.AddSingleton<JobsMedical.Web.Services.Scraping.IListingSource,
+    JobsMedical.Web.Services.Scraping.WebsiteListingSource>();
 builder.Services.AddScoped<JobsMedical.Web.Services.Scraping.ListingArchiver>();
 builder.Services.AddScoped<JobsMedical.Web.Services.Scraping.IngestionService>();
 builder.Services.AddHostedService<JobsMedical.Web.Services.Scraping.IngestionWorker>();
@@ -79,8 +81,11 @@ using (var scope = app.Services.CreateScope())
 {
     var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
     db.Database.Migrate();
-    // Production seeds reference data only (no demo facilities/shifts); dev seeds the full board.
-    await SeedData.EnsureSeededAsync(db, app.Environment.IsDevelopment());
+    await SeedData.SeedReferenceAsync(db);   // cities/roles/districts always
+    // Demo board in Development, or whenever the admin has turned Demo Mode on.
+    var st = await scope.ServiceProvider
+        .GetRequiredService<JobsMedical.Web.Services.Scraping.SettingsService>().GetAsync();
+    if (app.Environment.IsDevelopment() || st.DemoMode) await SeedData.SeedDemoAsync(db);
     // Archive any listings that went stale while the app was down.
     await scope.ServiceProvider
         .GetRequiredService<JobsMedical.Web.Services.Scraping.ListingArchiver>()