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/model/upload_stats.go

@@ -0,0 +1,28 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package model
+
+import "time"
+
+// Upload stats dimension keys stored in w_upload_stats.dimension.
+const (
+	UploadStatDimensionTotal    = "total"
+	UploadStatDimensionType     = "type"
+	UploadStatDimensionCategory = "category"
+	UploadStatDimensionTrend    = "trend"
+)
+
+// UploadStat stores incremental upload statistics keyed by dimension and stat_key.
+type UploadStat struct {
+	Dimension string    `json:"dimension" gorm:"primaryKey;size:32;not null"`
+	StatKey   string    `json:"stat_key" gorm:"primaryKey;size:64;not null;default:''"`
+	FileCount int64     `json:"file_count" gorm:"not null;default:0"`
+	FileSize  int64     `json:"file_size" gorm:"not null;default:0"`
+	UpdatedAt time.Time `json:"updated_at" gorm:"autoUpdateTime"`
+}
+
+// TableName returns the upload stats table name.
+func (UploadStat) TableName() string {
+	return "w_upload_stats"
+}