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/model/meta.go

@@ -1,8 +1,11 @@
 package model
 
 type Meta struct {
-	Filename  string `json:"filename" mapstructure:"filename"`
-	CreatedAt int64  `json:"created_at" mapstructure:"created_at"`
-	Size      int64  `json:"size" mapstructure:"size"`
-	Uploader  string `json:"uploader" mapstructure:"uploader"`
+	Filename     string `json:"filename"      mapstructure:"filename"`
+	CreatedAt    int64  `json:"created_at"    mapstructure:"created_at"`
+	Size         int64  `json:"size"          mapstructure:"size"`
+	Uploader     string `json:"uploader"      mapstructure:"uploader"`
+	MaxDownloads int    `json:"max_downloads" mapstructure:"max_downloads"`
+	ExpiresAt    int64  `json:"expires_at"    mapstructure:"expires_at"`
+	Downloads    int    `json:"downloads"     mapstructure:"downloads"`
 }