perf(storage): add config caching and shared HTTP connection pool

- Implement thread-safe local config caching with a 5s TTL check.
- Reuse backend client singletons in storage.Active and storage.ForDriver.
- Reset in-memory config cache on configuration saves and updates.
- Create internal/httppool package to manage shared HTTP transports.
- Configure WebDAV client and CDN retrieval to reuse the shared pool.
- Add unit tests for httppool and storage caching behaviors.

internal/storage/storage_test.go

@@ -0,0 +1,82 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package storage
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"testing"
+	"time"
+)
+
+func TestStorageCache(t *testing.T) {
+	// 1. Reset cache
+	ResetCache()
+
+	if activeConfigJSON != "" || activeDriver != "" || activeBackend != nil || !lastChecked.IsZero() {
+		t.Fatal("ResetCache did not clear cache variables")
+	}
+
+	// 2. Set up cache manually
+	expectedConfig := Config{
+		Driver: DriverLocal,
+		Local:  LocalConfig{Root: "/tmp/wavelet-test"},
+	}
+	cfgJSON, err := json.Marshal(expectedConfig)
+	if err != nil {
+		t.Fatalf("Marshal config failed: %v", err)
+	}
+
+	cacheMutex.Lock()
+	activeConfigJSON = string(cfgJSON)
+	lastChecked = time.Now()
+	cacheMutex.Unlock()
+
+	// 3. Call LoadConfig and verify it loads from cache (doesn't hit database, which would fail/panic because DB is not initialized)
+	ctx := context.Background()
+	loadedCfg, err := LoadConfig(ctx)
+	if err != nil {
+		t.Fatalf("LoadConfig failed: %v", err)
+	}
+
+	if loadedCfg.Driver != expectedConfig.Driver || loadedCfg.Local.Root != expectedConfig.Local.Root {
+		t.Errorf("Loaded config %+v, expected %+v", loadedCfg, expectedConfig)
+	}
+
+	// 4. Test Active() returns cached driver and backend
+	mockBnd := &functionBackend{
+		put:    func(context.Context, string, io.Reader, int64, string) error { return nil },
+		get:    func(context.Context, string) (*Object, error) { return nil, nil },
+		delete: func(context.Context, string) error { return nil },
+	}
+
+	cacheMutex.Lock()
+	activeBackend = mockBnd
+	activeDriver = DriverLocal
+	cacheMutex.Unlock()
+
+	drv, bnd, err := Active(ctx)
+	if err != nil {
+		t.Fatalf("Active failed: %v", err)
+	}
+	if drv != DriverLocal || bnd != mockBnd {
+		t.Errorf("Active returned driver %v, backend %v; expected %v, %v", drv, bnd, DriverLocal, mockBnd)
+	}
+
+	// 5. Test ForDriver returns cached backend
+	bnd2, err := ForDriver(ctx, DriverLocal)
+	if err != nil {
+		t.Fatalf("ForDriver failed: %v", err)
+	}
+	if bnd2 != mockBnd {
+		t.Errorf("ForDriver returned backend %v, expected %v", bnd2, mockBnd)
+	}
+
+	// 6. Test ResetCache again
+	ResetCache()
+	if activeConfigJSON != "" || activeDriver != "" || activeBackend != nil || !lastChecked.IsZero() {
+		t.Fatal("ResetCache did not clear cache variables after setting them")
+	}
+}