perf(upload): optimize file hot paths and incremental admin stats

- Use RWMutex for disk cache reads and singleflight for WebP cache misses
- Cache migration read-only state and file access whitelist with pub/sub invalidation
- Add w_upload_stats incremental counters updated on upload/delete
- Add w_uploads composite indexes and goose backfill migrations
- Document performance analysis in docs/PERFORMANCE.md
- Fix RegisterCustomRoutes to accept apiV1Router parameter

internal/apps/upload/stats_counter_test.go

@@ -0,0 +1,72 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package upload
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/Rain-kl/Wavelet/internal/db"
+	"github.com/Rain-kl/Wavelet/internal/model"
+	"github.com/Rain-kl/Wavelet/internal/testhelper"
+)
+
+func TestApplyUploadStatsAddAndRemove(t *testing.T) {
+	_, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+	ctx := context.Background()
+
+	upload := &model.Upload{
+		ID:        42001,
+		FileSize:  128,
+		MimeType:  "image/png",
+		Extension: "png",
+		Type:      "avatar",
+		Status:    model.UploadStatusUsed,
+		CreatedAt: time.Now(),
+	}
+	if err := ApplyUploadStatsAdd(ctx, upload); err != nil {
+		t.Fatalf("ApplyUploadStatsAdd returned error: %v", err)
+	}
+
+	stats, err := loadUploadStats(ctx)
+	if err != nil {
+		t.Fatalf("loadUploadStats returned error: %v", err)
+	}
+	if stats.TotalCount != 1 || stats.TotalSize != 128 {
+		t.Fatalf("unexpected total stats: count=%d size=%d", stats.TotalCount, stats.TotalSize)
+	}
+
+	if err := ApplyUploadStatsRemove(ctx, upload); err != nil {
+		t.Fatalf("ApplyUploadStatsRemove returned error: %v", err)
+	}
+
+	stats, err = loadUploadStats(ctx)
+	if err != nil {
+		t.Fatalf("loadUploadStats after remove returned error: %v", err)
+	}
+	if stats.TotalCount != 0 || stats.TotalSize != 0 {
+		t.Fatalf("expected zeroed total stats, got count=%d size=%d", stats.TotalCount, stats.TotalSize)
+	}
+}
+
+type uploadStatsSnapshot struct {
+	TotalCount int64
+	TotalSize  int64
+}
+
+func loadUploadStats(ctx context.Context) (uploadStatsSnapshot, error) {
+	var rows []model.UploadStat
+	if err := db.DB(ctx).Where("dimension = ?", model.UploadStatDimensionTotal).Find(&rows).Error; err != nil {
+		return uploadStatsSnapshot{}, err
+	}
+	if len(rows) == 0 {
+		return uploadStatsSnapshot{}, nil
+	}
+	return uploadStatsSnapshot{
+		TotalCount: rows[0].FileCount,
+		TotalSize:  rows[0].FileSize,
+	}, nil
+}