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

scripts/send-reminders.ts

@@ -0,0 +1,90 @@
+// @ts-nocheck
+import { PrismaClient } from '../src/generated/prisma/client';
+
+const prisma = new PrismaClient();
+
+const MATTERMOST_WEBHOOK_URL = process.env.MATTERMOST_WEBHOOK_URL;
+
+if (!MATTERMOST_WEBHOOK_URL) {
+  console.error('MATTERMOST_WEBHOOK_URL is not set');
+  process.exit(1);
+}
+
+async function sendToMattermost(message: string): Promise<boolean> {
+  try {
+    const payload = {
+      text: `⏰ Reminder: ${message}`,
+    };
+
+    const response = await fetch(MATTERMOST_WEBHOOK_URL, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify(payload),
+    });
+
+    if (!response.ok) {
+      console.error(`Mattermost webhook failed: ${response.status} ${response.statusText}`);
+      return false;
+    }
+
+    console.log(`Sent reminder to Mattermost: ${message}`);
+    return true;
+  } catch (error) {
+    console.error('Error sending to Mattermost:', error);
+    return false;
+  }
+}
+
+async function main() {
+  const now = new Date();
+  console.log(`Checking for due reminders at ${now.toISOString()}...`);
+
+  // 1. Find unsent standalone reminders that are due
+  const dueReminders = await prisma.reminder.findMany({
+    where: {
+      sent: false,
+      dueDate: { lte: now },
+    },
+    orderBy: { dueDate: 'asc' },
+  });
+
+  for (const reminder of dueReminders) {
+    const sent = await sendToMattermost(reminder.message);
+    if (sent) {
+      await prisma.reminder.update({
+        where: { id: reminder.id },
+        data: { sent: true },
+      });
+    }
+  }
+
+  // 2. Find unsent task reminders that are due
+  const dueTaskReminders = await prisma.taskReminder.findMany({
+    where: {
+      sent: false,
+      reminder: { lte: now },
+    },
+    orderBy: { reminder: 'asc' },
+    include: { task: true },
+  });
+
+  for (const tr of dueTaskReminders) {
+    const message = `[${tr.task.title}]${tr.task.description ? ` - ${tr.task.description}` : ''}`;
+    const sent = await sendToMattermost(message);
+    if (sent) {
+      await prisma.taskReminder.update({
+        where: { id: tr.id },
+        data: { sent: true },
+      });
+    }
+  }
+
+  await prisma.$disconnect();
+  console.log('Done.');
+}
+
+main().catch((error) => {
+  console.error('Cron worker error:', error);
+  prisma.$disconnect();
+  process.exit(1);
+});