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/config.py

@@ -22,6 +22,10 @@ class Config:
     PERMANENT_SESSION_LIFETIME = 28800  # 8 hours
     WTF_CSRF_TIME_LIMIT = 3600  # 1 hour
 
+    # Station identity: each deployed client container sets this to the station
+    # code it belongs to. Empty/None means "not configured".
+    STATION_CODE: str | None = os.getenv("STATION_CODE") or None
+
     # Babel i18n
     BABEL_DEFAULT_LOCALE = "it"
     BABEL_DEFAULT_TIMEZONE = "Europe/Rome"