Split Site:Audience into Site:ExternalUrl + Site:CorsAllowedOrigins

The Site:Audience setting was conflating two distinct concepts: an OAuth
JWT audience (a single resource identifier) and a CORS allow-list (an
array of origins). Collapsing them caused several latent bugs:
- OAuth/JWT validation expected a single string while CORS WithOrigins
  accepts an array.
- Password-reset callback URLs and OAuth client RedirectUri/Origin were
  being built from what was meant to be an audience identifier, not a
  base URL.
- Yavsc.Org's main CORS policy was hardcoded to '*', with no way to
  restrict it without code changes.

Changes:
- SiteSettings.Audience (string) replaced with CorsAllowedOrigins
  (IList<string>).
- OAuth JWT Authority still reads Site:Authority; Audience now reads
  Site:ExternalUrl (Org only; Api/Blogs use ValidateAudience=false).
- MailSender and AccountController build reset-callback URLs from
  Site:ExternalUrl.
- ClientController uses Site:ExternalUrl for OAuth RedirectUri/Origin
  defaults on newly created clients.
- Yavsc.Api and Yavsc.Blogs now read CORS origins from
  Site:CorsAllowedOrigins instead of hardcoded URLs.

Add shared AddYavscCors / AddYavscJwtBearer extension methods in
Yavsc.Server/Helpers/ServiceExtensions.cs to enforce a single
configuration contract across all runtime services (Api, Blogs, Org).
Fails closed when CorsAllowedOrigins is empty; fails fast at startup
when Site:Authority is missing.

Remove obsolete ConfigurationHelpers.GetAudience (no remaining callers).

Local appsettings-*.json files (which carry deployment-specific values
and are gitignored) must be updated to add Site:CorsAllowedOrigins.

src/Yavsc.Api/Program.cs

@@ -64,28 +64,12 @@ internal class Program
                         .RequireClaim(JwtClaimTypes.Scope, new string[] { "com" });
                 });
             })
-            .AddCors(options =>
-            {
-                // this defines a CORS policy called "default"
-                options.AddPolicy("default", policy =>
-                {
-                    policy.WithOrigins("https://localhost:5003")
-                        .AllowAnyHeader()
-                        .AllowAnyMethod();
-                });
-            })
+            .AddYavscCors(builder.Configuration)
             .AddControllers();
 
         // accepts any access token issued by identity server
-        var authenticationBuilder = services.AddAuthentication("Bearer")
-                 .AddJwtBearer("Bearer", options =>
-                 {
-                     options.IncludeErrorDetails = true;
-                     options.Authority = "https://localhost:5001";
-                     options.TokenValidationParameters =
-                         new() { ValidateAudience = false, RoleClaimType = YavscConstants.RoleClaimType };
-                     options.MapInboundClaims = true;
-                 });
+        services.AddAuthentication("Bearer")
+                 .AddYavscJwtBearer(builder.Configuration);
 
         services.AddDbContext<ApplicationDbContext>(options =>