feat: allow reverse proxies

.env.example

@@ -2,6 +2,11 @@ APP_NAME=Contínuos CRR
 SECRET_KEY=replace-with-a-long-random-value
 DATABASE_URL=sqlite:///./data/bar_rota.db
 BASE_URL=http://localhost:8000
+
+# Behind a reverse proxy: the public address of the app, used in notification e-mails.
+# ROOT_PATH is the prefix the proxy serves us under, e.g. /crr for https://host/crr/.
+# Leave it empty when the app is at the root of the domain.
+ROOT_PATH=
 PORT=8000
 
 # Host directory bind-mounted at /app/data (used by docker compose only).
@@ -25,5 +30,5 @@ SMTP_STARTTLS=true
 # Reminder command looks this many days ahead.
 REMINDER_DAYS_AHEAD=1
 
-# Set true when the app is served only over HTTPS.
+# Set true when the app is served only over HTTPS (always true behind a TLS proxy).
 SESSION_HTTPS_ONLY=false