perf(cap): add runtime settings snapshot for dynamic config

Replace per-request GetByKey calls with a singleflight-backed
RuntimeSettings snapshot loaded via ListSystemConfigsByKeys.
Invalidate snapshot on admin cap_* writes and via Redis pub/sub.
Simplify VerifyMiddleware and update PERFORMANCE.md status.

internal/apps/cap/testhelper_hook.go

@@ -0,0 +1,10 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package cap
+
+import "github.com/Rain-kl/Wavelet/internal/testhelper"
+
+func init() {
+	testhelper.RegisterCleanup(ResetRuntimeSettingsForTest)
+}