perf(backend): optimize ban lookups and prune upload group map

- Optimize the ban matching middleware by using a read-only transaction (`db.View`) for the initial scan, avoiding the single bbolt write lock on every request when no ban matches.
- Implement periodic pruning of stale entries in the upload grouper map to prevent unbounded memory growth over time.
- Avoid redundant parsing of the `max_days` form value in the upload handler.

backend/libs/handlers/upload.go

@@ -80,7 +80,8 @@ func (a *App) Upload(w http.ResponseWriter, r *http.Request) {
 	// Unlimited expiry: admins, or users whose effective MaxDays is negative.
 	unlimitedExpiry := isAdminUpload || effectivePolicy.MaxDays < 0
 
-	maxDays := parseInt(r.FormValue("max_days"))
+	rawMaxDays := parseInt(r.FormValue("max_days"))
+	maxDays := rawMaxDays
 	if maxDays <= 0 {
 		maxDays = 7
 		if effectivePolicy.MaxDays > 0 && effectivePolicy.MaxDays < maxDays {
@@ -96,7 +97,7 @@ func (a *App) Upload(w http.ResponseWriter, r *http.Request) {
 	expiresMinutes := parseInt(r.FormValue("expires_minutes"))
 	// A negative expires_minutes (or max_days) is the "never expires" request.
 	// Only honour it for unlimited uploaders; otherwise it's an invalid value.
-	if expiresMinutes < 0 || parseInt(r.FormValue("max_days")) < 0 {
+	if expiresMinutes < 0 || rawMaxDays < 0 {
 		if !unlimitedExpiry {
 			a.logger.Warn("upload rejected unlimited expiration", "source", "user-upload", "severity", "warn", "code", 4133, "ip", uploadClientIP(r), "user_id", user.ID)
 			helpers.WriteJSONError(w, http.StatusRequestEntityTooLarge, fmt.Sprintf("expiration cannot exceed %d days", effectivePolicy.MaxDays))