feat(worker): 实现集群限流与自适应负载

保留平台模型 RPM、TPM 和并发策略语义，增加 PostgreSQL 集群级租约、饱和候选重选和多平台自动负载，避免突发任务固定等待首个平台。\n\n新增 Worker 实时负载采样、自适应 active/heavy 容量、心跳与管理端指标，并扩展本地 acceptance runner，覆盖三 Worker、同模型三平台 2/4/6 并发和 48 个带图视频突发任务。\n\n验证：go test ./...、go vet ./...、PostgreSQL 跨 Store 集成测试、gofmt、bash -n、ShellCheck 及本地集群 provider-burst 验收通过；48/48 成功，无越限、重复提交、重复计费、重复回调或终态资源泄漏。

apps/api/internal/store/async_worker_capacity.go

@@ -6,19 +6,28 @@ import (
 )
 
 type AsyncWorkerCapacitySnapshot struct {
-	Capacity        int
-	GlobalCapacity  int
-	Desired         int
-	HardLimit       int
-	Capped          bool
-	EnabledModels   int
-	UnlimitedModels int
-	EnabledGroups   int
-	UnlimitedGroups int
-	ModelDesired    int
-	GroupDesired    int
-	ActiveInstances int
-	InstanceID      string
+	Capacity             int
+	GlobalCapacity       int
+	Desired              int
+	HardLimit            int
+	Capped               bool
+	EnabledModels        int
+	UnlimitedModels      int
+	EnabledGroups        int
+	UnlimitedGroups      int
+	ModelDesired         int
+	GroupDesired         int
+	ActiveInstances      int
+	InstanceID           string
+	LoadMode             string
+	LocalSafeCapacity    int
+	LocalHeavyCapacity   int
+	LocalActiveTasks     int
+	LocalPreparingTasks  int
+	LocalWaitingTasks    int
+	LocalFinalizingTasks int
+	LocalPressureState   string
+	LocalPressureReason  string
 }
 
 func (s *Store) AsyncWorkerCapacity(ctx context.Context, hardLimit int) (AsyncWorkerCapacitySnapshot, error) {