feat: due-date reminders

Every hour the API reminds whoever has a ticket, and whoever follows it,
the day before its due date, on the day and once when it is overdue, in
the app and by e-mail for those who asked. Cards on the board flag what
is due soon or late.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -64,6 +64,9 @@ services:
       WEB_URL: ${WEB_URL:-}
       # Outgoing webhooks to private addresses (off: they could reach internal services).
       WEBHOOKS_ALLOW_PRIVATE: ${WEBHOOKS_ALLOW_PRIVATE:-false}
+      # Due-date reminders, checked every hour.
+      REMINDERS_ENABLED: ${REMINDERS_ENABLED:-true}
+      REMINDERS_TIMEZONE: ${REMINDERS_TIMEZONE:-Europe/Madrid}
     volumes:
       # Files attached to tickets: lose this volume and the tickets point at nothing.
       - attachments-data:/data/attachments