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>

.env.example

@@ -48,6 +48,10 @@ WEB_URL=http://localhost:4200
 # Outgoing webhooks refuse hosts on this machine or its private network unless true.
 WEBHOOKS_ALLOW_PRIVATE=false
 
+# Due-date reminders (hourly). The day changes at midnight in this time zone.
+REMINDERS_ENABLED=true
+REMINDERS_TIMEZONE=Europe/Madrid
+
 # Docker builds only: token of Pireon's Nexus for @pi/ui (build secret, never stored in an image).
 # Locally: export NEXUS_TOKEN=$(grep '^//nexus.pireon.dev/repository/npm-releases/:_authToken=' ~/.npmrc | cut -d= -f2-)
 NEXUS_TOKEN=