feat(admin): add super admin initialization endpoint

Add POST /api/v1/admin/setup-super-admin endpoint to initialize the first super admin.
The endpoint can only be used once - after a super admin exists, subsequent requests
are rejected. Requires valid setup_secret configured via APP_SETUP_SECRET environment
variable or setup_secret in config file.

internal/wire/handler.go

@@ -43,6 +43,7 @@ var HandlerSet = wire.NewSet(
 	ProvideScheduleHandler,
 	ProvideWSHandler,
 	ProvideAdminVerificationHandler,
+	ProvideSetupHandler,
 )
 
 func ProvideUserHandler(
@@ -108,3 +109,9 @@ func ProvideAdminVerificationHandler(
 ) *handler.AdminVerificationHandler {
 	return handler.NewAdminVerificationHandler(adminVerificationService, userRepo)
 }
+
+func ProvideSetupHandler(
+	setupService service.SetupService,
+) *handler.SetupHandler {
+	return handler.NewSetupHandler(setupService)
+}