feat: add reminder system with cron scheduler and fix timezone formatting

- Add reminders to Prisma schema (Reminder model, TaskReminder relation)
- Add /api/reminders endpoint and cron send-reminders.ts script
- Add reminder fields to NewTaskModal and EditTaskModal components
- Fix reminder datetime serialization: use toISOString().slice(0,16) for
  UTC-safe YYYY-MM-DDTHH:mm format compatible with datetime-local inputs
- Update Dockerfile to install tsx for cron container
- Add AGENTS.md with project conventions
- Update docker-compose.yml with cron service and build context

docker-compose.yml

@@ -27,11 +27,29 @@ services:
       postgres:
         condition: service_healthy
     ports:
-      - "3070:3000"
+      - "3000:3000"
     environment:
       DATABASE_URL: postgresql://vixtix:vixtix_secret@postgres:5432/vixtix
       DB_HOST: postgres
       DB_PORT: 5432
 
+  vixtix-cron:
+    image: vixtix:latest
+    container_name: vixtix-cron
+    restart: unless-stopped
+    depends_on:
+      postgres:
+        condition: service_healthy
+    environment:
+      DATABASE_URL: postgresql://vixtix:vixtix_secret@postgres:5432/vixtix
+      MATTERMOST_WEBHOOK_URL: ${MATTERMOST_WEBHOOK_URL}
+    entrypoint: ["sh", "-c"]
+    command:
+      - |
+        while true; do
+          npx tsx /app/scripts/send-reminders.ts
+          sleep 60
+        done
+
 volumes:
   vixtix-data: