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/workerload/sampler_test.go

@@ -0,0 +1,34 @@
+package workerload
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestSystemSamplerReadsCgroupV2(t *testing.T) {
+	root := t.TempDir()
+	writeSamplerFixture(t, filepath.Join(root, "memory.current"), "50\n")
+	writeSamplerFixture(t, filepath.Join(root, "memory.max"), "100\n")
+	writeSamplerFixture(t, filepath.Join(root, "cpu.max"), "100000 100000\n")
+	writeSamplerFixture(t, filepath.Join(root, "cpu.stat"), "usage_usec 100000\nnr_throttled 1\n")
+	sampler := NewSystemSamplerAt(root)
+	first := sampler.Sample(2, 10)
+	if first.MemoryCurrentBytes != 50 || first.MemoryLimitBytes != 100 || first.DBConnections != 2 {
+		t.Fatalf("first sample=%+v", first)
+	}
+	time.Sleep(10 * time.Millisecond)
+	writeSamplerFixture(t, filepath.Join(root, "cpu.stat"), "usage_usec 105000\nnr_throttled 2\n")
+	second := sampler.Sample(3, 10)
+	if second.CPUUtilization <= 0 || !second.CPUThrottled {
+		t.Fatalf("second sample=%+v", second)
+	}
+}
+
+func writeSamplerFixture(t *testing.T, path, value string) {
+	t.Helper()
+	if err := os.WriteFile(path, []byte(value), 0o600); err != nil {
+		t.Fatal(err)
+	}
+}