feat: add payment management improvements and reminder emails

- Add option to approve/reject payments without sending notification emails
  (checkbox in review popup, default enabled)
- Add payment reminder email template and send functionality
- Track when reminder emails are sent (reminderSentAt field)
- Display reminder sent timestamp in payment review popup
- Make payment review popup scrollable for better UX
- Add payment-reminder template to email system (available in admin emails)

backend/src/db/migrate.ts

@@ -209,6 +209,9 @@ async function migrate() {
     try {
       await (db as any).run(sql`ALTER TABLE payments ADD COLUMN payer_name TEXT`);
     } catch (e) { /* column may already exist */ }
+    try {
+      await (db as any).run(sql`ALTER TABLE payments ADD COLUMN reminder_sent_at TEXT`);
+    } catch (e) { /* column may already exist */ }
 
     // Invoices table
     await (db as any).run(sql`
@@ -577,6 +580,9 @@ async function migrate() {
     try {
       await (db as any).execute(sql`ALTER TABLE payments ADD COLUMN payer_name VARCHAR(255)`);
     } catch (e) { /* column may already exist */ }
+    try {
+      await (db as any).execute(sql`ALTER TABLE payments ADD COLUMN reminder_sent_at TIMESTAMP`);
+    } catch (e) { /* column may already exist */ }
 
     // Invoices table
     await (db as any).execute(sql`