feat: add gateway client with env-driven assembly

Includes config aggregation for multi-source env keys, from_env and
from_settings factories with explicit shared-backend injection,
gather_bounded, top-level exports, tightened import-linter layers with
the gate removed from the Makefile, and the finalized .env.example.

src/polygateway/backends/memory/limiter.py

@@ -27,7 +27,9 @@ _WINDOW_S = 60.0
 class _MemoryPermit:
     """入场许可;release/settle 幂等(CHS _RedisPermit 同款 flag 语义)。"""
 
-    def __init__(self, limiter: InMemoryLimiter, source: str, lease_id: str, est: int, window: int) -> None:
+    def __init__(
+        self, limiter: InMemoryLimiter, source: str, lease_id: str, est: int, window: int
+    ) -> None:
         self._limiter = limiter
         self._source = source
         self._lease_id = lease_id