perf(gateway): 流式处理代码层性能优化

- 将 spawn 内 buffered_body 从 Vec<u8> 改为 VecDeque<u8>, drain 前端从 O(n) 变 O(1)
- 消除 prefetched body/chunks 的不必要 clone, 改为直接 move
- Pending/Streaming 非终态候选状态写入改为 tokio::spawn fire-and-forget
- 连接池默认 max_connections 从 30 提升到 50

crates/aether-data/src/postgres/pool.rs

@@ -21,7 +21,7 @@ impl Default for PostgresPoolConfig {
         Self {
             database_url: String::new(),
             min_connections: 1,
-            max_connections: 30,
+            max_connections: 50,
             acquire_timeout_ms: 3_000,
             idle_timeout_ms: 60_000,
             max_lifetime_ms: 30 * 60_000,