chore(gateway): 调小 Postgres 连接池默认值

- max_connections 100 → 20
- idle_timeout 60s → 30s
- .env.example 补充连接池相关变量注释

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

@@ -21,9 +21,9 @@ impl Default for PostgresPoolConfig {
         Self {
             database_url: String::new(),
             min_connections: 1,
-            max_connections: 100,
+            max_connections: 20,
             acquire_timeout_ms: 10_000,
-            idle_timeout_ms: 60_000,
+            idle_timeout_ms: 30_000,
             max_lifetime_ms: 30 * 60_000,
             statement_cache_capacity: 100,
             require_ssl: false,