fix(cluster): 修复身份事务阻塞与 Worker 容量抖动

将身份协调和安全事件心跳改为 PostgreSQL 单 Leader 执行，并从独立 Worker 进程中移除身份运行时，避免多副本重复写同一状态。

为安全事件事务增加锁等待、空闲事务超时及独立回滚上下文；Worker 需连续丢失六次心跳后才判定失效，降低跨节点抖动导致的容量反复扩缩。

验证：Go 全量测试、go vet、Race 聚焦测试、PostgreSQL 18 Leader/安全事件/Worker 分配集成测试及迁移测试通过。

apps/api/internal/store/leadership_integration_test.go

@@ -0,0 +1,52 @@
+package store
+
+import (
+	"context"
+	"os"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestPostgresLeadershipIsExclusiveAndRecoverable(t *testing.T) {
+	databaseURL := strings.TrimSpace(os.Getenv("AI_GATEWAY_TEST_DATABASE_URL"))
+	if databaseURL == "" {
+		t.Skip("set AI_GATEWAY_TEST_DATABASE_URL to run PostgreSQL leadership integration tests")
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+	defer cancel()
+	first, err := Connect(ctx, databaseURL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer first.Close()
+	second, err := Connect(ctx, databaseURL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer second.Close()
+
+	leader, acquired, err := first.TryAcquireSecurityEventHeartbeatLeadership(ctx)
+	if err != nil || !acquired {
+		t.Fatalf("first leadership acquired=%v err=%v", acquired, err)
+	}
+	defer leader.Release()
+	if err := leader.KeepAlive(ctx); err != nil {
+		t.Fatalf("leadership keepalive: %v", err)
+	}
+	blocked, acquired, err := second.TryAcquireSecurityEventHeartbeatLeadership(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if acquired {
+		blocked.Release()
+		t.Fatal("second store acquired leadership while first store held it")
+	}
+
+	leader.Release()
+	replacement, acquired, err := second.TryAcquireSecurityEventHeartbeatLeadership(ctx)
+	if err != nil || !acquired {
+		t.Fatalf("replacement leadership acquired=%v err=%v", acquired, err)
+	}
+	replacement.Release()
+}