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

src/app/api/reminders/route.ts

@@ -0,0 +1,62 @@
+import { NextRequest, NextResponse } from 'next/server';
+import prisma from '@/lib/db';
+
+export async function GET(request: NextRequest) {
+  const { searchParams } = new URL(request.url);
+  const sent = searchParams.get('sent');
+
+  try {
+    const where: Record<string, unknown> = {};
+    if (sent !== null) where.sent = sent === 'true';
+
+    const reminders = await prisma.reminder.findMany({
+      where,
+      orderBy: [{ dueDate: 'asc' }],
+    });
+
+    return NextResponse.json(
+      reminders.map((r) => ({
+        ...r,
+        dueDate: r.dueDate.toISOString(),
+      }))
+    );
+  } catch (error) {
+    console.error('Error fetching reminders:', error);
+    return NextResponse.json({ error: 'Failed to fetch reminders' }, { status: 500 });
+  }
+}
+
+export async function POST(request: NextRequest) {
+  let body;
+  try {
+    body = await request.json();
+  } catch {
+    return NextResponse.json({ error: 'Invalid JSON' }, { status: 400 });
+  }
+
+  const { message, dueDate } = body;
+
+  if (!message?.trim()) {
+    return NextResponse.json({ error: 'Message is required' }, { status: 400 });
+  }
+  if (!dueDate) {
+    return NextResponse.json({ error: 'dueDate is required' }, { status: 400 });
+  }
+
+  try {
+    const reminder = await prisma.reminder.create({
+      data: {
+        message: message.trim(),
+        dueDate: new Date(dueDate),
+      },
+    });
+
+    return NextResponse.json({
+      ...reminder,
+      dueDate: reminder.dueDate.toISOString(),
+    });
+  } catch (error) {
+    console.error('Error creating reminder:', error);
+    return NextResponse.json({ error: 'Failed to create reminder' }, { status: 500 });
+  }
+}