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/middleware/cache.py

@@ -111,8 +111,12 @@ class CacheMW:
             fields = {k: v for k, v in data.items() if k in _RESPONSE_FIELDS}
             structured_data = self._rebuild_structured(fields.get("content", ""), request)
             fields.update(
-                cache_hit=True, latency_ms=0, ttft_ms=None, max_inter_token_ms=None,
-                call_id=str(uuid.uuid4()), structured_data=structured_data,
+                cache_hit=True,
+                latency_ms=0,
+                ttft_ms=None,
+                max_inter_token_ms=None,
+                call_id=str(uuid.uuid4()),
+                structured_data=structured_data,
             )
             return LLMResponse(**fields)
         except Exception as exc: