feat: add download limit and expiry control per upload (v0.7.0)

Backend:
- model/meta.go: add MaxDownloads, ExpiresAt, Downloads fields
- opt/var.go: add X-Max-Downloads, X-Expires-In header constants; MinExpiresIn=30s, defaults
- controller/meta.go: New() accepts maxDownloads+expiresIn; CheckAndIncrDownload() validates expiry/limit and increments counter atomically; periodic cleanup for expired files
- handler/share.go: Fetch uses CheckAndIncrDownload (returns 410 on expired/limit exceeded); ShareNew and ShareAPIUpload read X-Max-Downloads/X-Expires-In headers

Frontend:
- upload.ts: UploadSettings interface; pass X-Max-Downloads and X-Expires-In headers on upload init
- panel-left.tsx: collapsible "高级设置" panel with download count (0-999) and expiry (1-24h) controls; show settings summary on upload success card

🤖 Generated with [Qoder][https://qoder.com]

internal/opt/var.go

@@ -3,9 +3,18 @@ package opt
 import "path/filepath"
 
 const (
-	Meta       = ".meta."
-	HeaderSize = "X-File-Size"
-	CodeLength = 8
+	Meta              = ".meta."
+	HeaderSize        = "X-File-Size"
+	HeaderMaxDownload = "X-Max-Downloads"
+	HeaderExpiresIn   = "X-Expires-In"
+	CodeLength        = 8
+
+	// MinExpiresIn is the minimum allowed expiry in seconds (30s for testing).
+	MinExpiresIn = 30
+	// DefaultExpiresIn is the default expiry in seconds (8 hours).
+	DefaultExpiresIn = 8 * 3600
+	// DefaultMaxDownloads is the default max download count (0 = unlimited).
+	DefaultMaxDownloads = 3
 )
 
 func FilePath(code string) string {