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.EmulatedEntrypoint/Controllers/AdminController.cs

@@ -16,8 +16,10 @@ using SVSim.EmulatedEntrypoint.Services;
 namespace SVSim.EmulatedEntrypoint.Controllers;
 
 /// <summary>
-/// Util endpoints for bootstrapping the dev environment. Anonymous-allowed today — security
-/// audit pending (don't expose these to the public internet).
+/// Util endpoints for bootstrapping the dev environment. Actions are gated by
+/// <see cref="RequireAdminSecretAttribute"/>: callers must send the shared secret from
+/// <c>Admin:ImportSecret</c> in the <c>X-Admin-Secret</c> header. Missing/blank config
+/// disables the endpoint (fail-closed).
 /// </summary>
 public class AdminController : SVSimController
 {
@@ -41,6 +43,7 @@ public class AdminController : SVSimController
     /// Only essential fields are imported today — extend as needed.
     /// </summary>
     [AllowAnonymous]
+    [RequireAdminSecret]
     [HttpPost("import_viewer")]
     public async Task<ActionResult<ImportViewerResponse>> ImportViewer(ImportViewerRequest request)
     {