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.Org/Controllers/Accounting/AccountController.cs

@@ -590,7 +590,7 @@ IHtmlLocalizerFactory htmlLocalizerFactory,
                        _siteSettings.Title,
                        callbackUrl,
                        _siteSettings.Slogan,
-                       _siteSettings.Audience));
+                       _siteSettings.ExternalUrl));
                     // No, wait for more than a login pass submission:
                     // do not await _signInManager.SignInAsync(user, isPersistent: false);
 
@@ -641,7 +641,7 @@ IHtmlLocalizerFactory htmlLocalizerFactory,
             this._localizer["ConfirmYourAccountTitle"],
             string.Format(this._localizer["ConfirmYourAccountBody"],
                   _siteSettings.Title, callbackUrl, _siteSettings.Slogan,
-                   _siteSettings.Audience));
+                   _siteSettings.ExternalUrl));
             return new EmailSentViewModel { EMail = user.Email, Sent = true, MessageId = res };
         }
 
@@ -654,7 +654,7 @@ IHtmlLocalizerFactory htmlLocalizerFactory,
             this._localizer["AccountEmailFactorTitle"],
             string.Format(this._localizer["AccountEmailFactorBody"],
                   _siteSettings.Title, callbackUrl, _siteSettings.Slogan,
-                   _siteSettings.Audience, code));
+                   _siteSettings.ExternalUrl, code));
             return new EmailSentViewModel { EMail = user.Email, Sent = true, MessageId = res }; ;
         }
         //