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/telemetry/sqlite.py

@@ -39,10 +39,24 @@ CREATE TABLE IF NOT EXISTS llm_calls (
 """
 
 _COLUMNS = (
-    "call_id", "parent_call_id", "session_id", "model", "provider", "source_name",
-    "messages", "response", "thinking", "prompt_tokens", "completion_tokens",
-    "usage_source", "latency_ms", "ttft_ms", "max_inter_token_ms", "cache_hit",
-    "error", "cost",
+    "call_id",
+    "parent_call_id",
+    "session_id",
+    "model",
+    "provider",
+    "source_name",
+    "messages",
+    "response",
+    "thinking",
+    "prompt_tokens",
+    "completion_tokens",
+    "usage_source",
+    "latency_ms",
+    "ttft_ms",
+    "max_inter_token_ms",
+    "cache_hit",
+    "error",
+    "cost",
 )
 
 _INSERT = (