feat(admin): shared-secret auth for /admin/import_viewer + Swagger authorize

Anonymous /admin/import_viewer was safe locally but exposed viewer-import surface
whenever the server was reachable off-box. Gate it on a shared secret carried in
X-Admin-Secret, sourced from Admin:ImportSecret in appsettings (override via
ADMIN__IMPORTSECRET env var). Fails closed: an unconfigured deployment 401s every
request (with a logged warning) instead of leaving the endpoint open.

Implementation is a plain IAuthorizationFilter attribute (RequireAdminSecret) that
runs before model binding — short-circuits the ImportViewerRequest deserialize on
unauthorized calls. Compare uses CryptographicOperations.FixedTimeEquals to avoid
timing signal. A companion Swagger IOperationFilter registers an ApiKey security
definition and attaches the requirement ONLY to actions carrying the attribute, so
the Swagger UI shows an Authorize dialog for admin endpoints without decorating
ordinary game routes with a padlock.

Tests: existing 27 AdminController tests routed through a new
SVSimTestFactory.CreateAdminClient() helper that bakes the header from
appsettings.Testing.json; added two negative-path tests
(missing_secret_header, wrong_secret) — 29/29 passing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.UnitTests/Infrastructure/SVSimTestFactory.cs

@@ -397,6 +397,20 @@ internal class SVSimTestFactory : WebApplicationFactory<Program>
         return client;
     }
 
+    /// <summary>
+    /// Shared secret baked into <c>appsettings.Testing.json</c> for <c>/admin/*</c> gating. Kept
+    /// here so tests can construct requests with or without a valid header via the same source.
+    /// </summary>
+    public const string AdminSecret = "test-admin-secret";
+
+    /// <summary>Convenience: bake the X-Admin-Secret header into a fresh client.</summary>
+    public HttpClient CreateAdminClient()
+    {
+        var client = CreateClient();
+        client.DefaultRequestHeaders.Add("X-Admin-Secret", AdminSecret);
+        return client;
+    }
+
     /// <summary>
     /// Inserts a deck for the viewer via the real <see cref="IDeckRepository.UpsertDeck"/>
     /// path. Picks the first seeded class/sleeve/leader-skin from the master tables; tests