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/telemetry.py

@@ -166,15 +166,19 @@ class TelemetryMW:
             response = await call_next(request)
         except (GatewayUnavailableError, GovernanceBackendError) as exc:
             await self._emitter.emit_terminal_failure(
-                request=request, call_id=str(uuid.uuid4()),
-                latency_ms=int((self._now() - started) * 1000), error=str(exc),
+                request=request,
+                call_id=str(uuid.uuid4()),
+                latency_ms=int((self._now() - started) * 1000),
+                error=str(exc),
             )
             raise
         except asyncio.CancelledError:
             # 尽力而为: 取消也留痕(§5.1 约定④);随后立即重抛
             await self._emitter.emit_terminal_failure(
-                request=request, call_id=str(uuid.uuid4()),
-                latency_ms=int((self._now() - started) * 1000), error="cancelled",
+                request=request,
+                call_id=str(uuid.uuid4()),
+                latency_ms=int((self._now() - started) * 1000),
+                error="cancelled",
             )
             raise
         if response.cache_hit: