feat[platform]: add fengling-api OAuth client for introspection

- Add RegisterApiClientAsync to seed OpenIddict applications
- Register fengling-api client for Console backend token introspection
- Client secret: fengling-api-secret (Confidential, with Introspection permission)

This enables Console backend to validate tokens via introspection endpoint.

Fengling.Platform.Infrastructure/SeedData.cs

@@ -85,6 +85,7 @@ public static class SeedData
         await RegisterCustomScopesAsync(scopeManager);
         await RegisterVbenConsoleClientAsync(applicationManager);
         await RegisterSwaggerClientAsync(applicationManager);
+        await RegisterApiClientAsync(applicationManager);
     }
 
     private static async Task RegisterCustomScopesAsync(IOpenIddictScopeManager scopeManager)
@@ -193,4 +194,26 @@ public static class SeedData
             }
         });
     }
+
+    private static async Task RegisterApiClientAsync(IOpenIddictApplicationManager applicationManager)
+    {
+        var existingClient = await applicationManager.FindByClientIdAsync("fengling-api");
+        if (existingClient != null)
+        {
+            return;
+        }
+
+        await applicationManager.CreateAsync(new OpenIddictApplicationDescriptor
+        {
+            ClientId = "fengling-api",
+            ClientSecret = "fengling-api-secret",
+            DisplayName = "Fengling API (Resource Server)",
+            ApplicationType = OpenIddictConstants.ApplicationTypes.Web,
+            ClientType = OpenIddictConstants.ClientTypes.Confidential,
+            Permissions =
+            {
+                OpenIddictConstants.Permissions.Endpoints.Introspection
+            }
+        });
+    }
 }