功能: 集成 rclone 高级传输特性 + 全 70+ 后端支持

1. 失败自动重试：rclone Pacer 指数退避，默认 10 次底层 HTTP 重试
2. 带宽限制：配置 bandwidth_limit + Settings 运行时可调
3. 上传实时进度：progressReader + LogHub SSE 推送字节级进度/速率
4. 存储空间查询：StorageAbout 可选接口，GetUsage 返回远端真实空间
5. 全 rclone 后端：backend/all 引入 70+ 后端，新增 rclone 存储类型，
   API 驱动的可搜索后端选择器 + 动态配置表单

server/internal/service/progress_reader.go

@@ -0,0 +1,52 @@
+package service
+
+import (
+	"io"
+	"sync/atomic"
+	"time"
+)
+
+// progressCallback 在每次读取时被调用，报告已读字节数和估算速率。
+type progressCallback func(bytesRead int64, speedBps float64)
+
+// progressReader 包装 io.Reader，定期通过回调报告传输进度。
+type progressReader struct {
+	reader    io.Reader
+	total     int64
+	read      atomic.Int64
+	callback  progressCallback
+	startTime time.Time
+	lastCall  time.Time
+	interval  time.Duration
+}
+
+func newProgressReader(reader io.Reader, total int64, callback progressCallback) *progressReader {
+	now := time.Now()
+	return &progressReader{
+		reader:    reader,
+		total:     total,
+		callback:  callback,
+		startTime: now,
+		lastCall:  now,
+		interval:  500 * time.Millisecond,
+	}
+}
+
+func (r *progressReader) Read(p []byte) (int, error) {
+	n, err := r.reader.Read(p)
+	if n > 0 {
+		current := r.read.Add(int64(n))
+		now := time.Now()
+		isFinal := err == io.EOF || (r.total > 0 && current >= r.total)
+		if isFinal || now.Sub(r.lastCall) >= r.interval {
+			r.lastCall = now
+			elapsed := now.Sub(r.startTime).Seconds()
+			speed := float64(0)
+			if elapsed > 0 {
+				speed = float64(current) / elapsed
+			}
+			r.callback(current, speed)
+		}
+	}
+	return n, err
+}