feat: introduce scheduler lanes for independent per-workload scheduling

Split the single plugin scheduler loop into independent per-lane
goroutines so that volume management, iceberg compaction, and lifecycle
operations never block each other.

Each lane has its own:
- Goroutine (laneSchedulerLoop)
- Wake channel for immediate scheduling
- Admin lock scope (e.g. "plugin scheduler:default")
- Configurable idle sleep duration
- Loop state tracking

Three lanes are defined:
- default: vacuum, volume_balance, ec_balance, erasure_coding, admin_script
- iceberg: iceberg_maintenance
- lifecycle: s3_lifecycle (new, handler coming in a later commit)

Job types are mapped to lanes via a hardcoded map with LaneDefault as
the fallback. The SchedulerJobTypeState and SchedulerStatus types now
include a Lane field for API consumers.

weed/admin/plugin/scheduler_status.go

@@ -251,6 +251,7 @@ func (r *Plugin) aggregateLaneLoopStates() schedulerLoopState {
 	return agg
 }
 
+
 // --- Per-lane loop state helpers ---
 
 func (r *Plugin) setLaneLoopState(ls *schedulerLaneState, jobType, phase string) {