feat(client): add STATION_CODE env var and config attribute

Reads STATION_CODE from the environment and exposes it as Config.STATION_CODE
(None when unset or empty). Adds the variable to .env.example with a
per-station deployment note, and covers both read and missing-key paths with
new pytest tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/tests/test_config_station.py

@@ -0,0 +1,17 @@
+"""Tests that STATION_CODE is loaded from env and exposed on the client Config class."""
+import importlib
+import os
+
+
+def test_station_code_read_from_env(monkeypatch):
+    monkeypatch.setenv("STATION_CODE", "ST-TEST")
+    import config
+    importlib.reload(config)
+    assert config.Config.STATION_CODE == "ST-TEST"
+
+
+def test_station_code_defaults_to_none_when_missing(monkeypatch):
+    monkeypatch.delenv("STATION_CODE", raising=False)
+    import config
+    importlib.reload(config)
+    assert config.Config.STATION_CODE is None