Add Jivosite live chat widget support

Resolves #3 — widget is loaded on every page via base.html when
JIVOSITE_WIDGET_ID env var is set. Centralized Jinja2Templates instance
in web/templates_env.py with jivosite_widget_id as a global.

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

web/config.py

@@ -10,6 +10,8 @@ class Settings(BaseSettings):
     EVOTOR_APP_ID: str = ""
     EVOTOR_WEBHOOK_SECRET: str = ""
 
+    JIVOSITE_WIDGET_ID: str = ""
+
     HEALTH_CHECK_INTERVAL_SECONDS: int = 600
     CATALOG_REFRESH_INTERVAL_SECONDS: int = 3600