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/worker_registry_test.go

@@ -42,3 +42,15 @@ func TestAllocateWorkerCapacitiesSupportsUnequalLimits(t *testing.T) {
 		t.Fatalf("allocations=%v global=%d, want 1/4 and 5", allocations, global)
 	}
 }
+
+func TestAllocateWorkerCapacitiesRedistributesFromPressuredWorker(t *testing.T) {
+	workers := []activeWorkerCapacity{
+		{InstanceID: "worker-a", CapacityLimit: 0},
+		{InstanceID: "worker-b", CapacityLimit: 6},
+		{InstanceID: "worker-c", CapacityLimit: 6},
+	}
+	allocations, global := allocateWorkerCapacities(workers, 8)
+	if global != 8 || allocations["worker-a"] != 0 || allocations["worker-b"] != 4 || allocations["worker-c"] != 4 {
+		t.Fatalf("allocations=%v global=%d, want 0/4/4 and 8", allocations, global)
+	}
+}