fix(routing): 避免路由探测耗尽数据库连接

将跨实例探测协调从持有会话级 advisory lock 改为带过期时间的数据库短租约，避免网络探测期间占用关键 PostgreSQL 连接。新增可配置的探测并发上限和真实 PostgreSQL 回归测试，确保租约生效期间连接池仍可全部获取。\n\n验证：go test ./... -count=1；go vet ./...；真实 PostgreSQL 17 集成测试；迁移安全检查。

apps/api/migrations/0106_route_probe_leases.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS gateway_route_probe_leases (
+  pool_id text NOT NULL REFERENCES gateway_execution_pools(pool_id) ON DELETE CASCADE,
+  route_profile_key text NOT NULL,
+  lease_token uuid NOT NULL,
+  expires_at timestamptz NOT NULL,
+  updated_at timestamptz NOT NULL DEFAULT now(),
+  PRIMARY KEY (pool_id, route_profile_key)
+);
+
+CREATE INDEX IF NOT EXISTS idx_gateway_route_probe_leases_expiry
+  ON gateway_route_probe_leases(expires_at);