CRITICAL: Check ALL task states for volume conflicts

Fix major scheduling bug where only active tasks were checked for conflicts.

Changes:
- Check PENDING tasks: Prevent scheduling if task is queued for same volume
- Check ASSIGNED/ACTIVE tasks: Prevent scheduling if task is running on same volume
- Check RECENT tasks: Prevent immediate re-scheduling on same volume after completion

This prevents dangerous scenarios like:
❌ Scheduling vacuum while another vacuum is pending on same volume
❌ Scheduling balance while erasure coding is queued for same volume
❌ Immediately re-scheduling failed tasks without cooldown period

Critical safety improvement ensuring comprehensive volume-level task isolation.

weed/admin/topology/internal.go

@@ -84,13 +84,28 @@ func (at *ActiveTopology) isDiskAvailableForVolume(disk *activeDisk, taskType Ta
 		return false
 	}
 
-	// Check for volume-specific conflicts
+	// Check for volume-specific conflicts in ALL task states:
+	// 1. Pending tasks (queued but not yet started)
+	for _, task := range disk.pendingTasks {
+		if at.areTasksConflicting(task, taskType, volumeID) {
+			return false
+		}
+	}
+
+	// 2. Assigned/Active tasks (currently running)
 	for _, task := range disk.assignedTasks {
 		if at.areTasksConflicting(task, taskType, volumeID) {
 			return false
 		}
 	}
 
+	// 3. Recent tasks (just completed - avoid immediate re-scheduling on same volume)
+	for _, task := range disk.recentTasks {
+		if at.areTasksConflicting(task, taskType, volumeID) {
+			return false
+		}
+	}
+
 	return true
 }