feat: add template for email

apps/api/templates/emails/README.md

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+# Email templates
+
+Each email the API sends is a pair of files in this folder:
+
+| File | Purpose |
+|---|---|
+| `<name>.html` | The email itself. Its `<title>` is used as the subject line. |
+| `<name>.txt` | Optional plain-text version, shown by mail clients that don't render HTML. |
+
+Templates are read from disk every time an email is sent, so edits apply without restarting the API.
+To keep them outside the image, mount a folder and point `EMAIL_TEMPLATES_DIR` at it.
+
+## Placeholders
+
+Write `{{name}}` to insert a value. Values are HTML-escaped in the `.html` file (and the subject),
+and inserted as-is in the `.txt` file. A placeholder without a value stops the email from being sent,
+so a typo never reaches a customer.
+
+### `payment-reminder`
+
+Sent by "Send reminders" to customers with an unpaid balance.
+
+| Placeholder | Example |
+|---|---|
+| `{{customerName}}` | Jan Peeters |
+| `{{venueName}}` | Café De Kroon |
+| `{{amount}}` | €12.50 |
+| `{{itemCount}}` | 3 |
+
+### `test-email`
+
+Sent by the test button next to a customer's email address.
+
+| Placeholder | Example |
+|---|---|
+| `{{customerName}}` | Jan Peeters |
+| `{{venueName}}` | Café De Kroon |

apps/api/templates/emails/payment-reminder.html

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+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>Payment reminder for your tab at {{venueName}}</title>
+  </head>
+  <body>
+    <p>Hi {{customerName}},</p>
+    <p>This is a friendly reminder that you have an open tab of <strong>{{amount}}</strong> at {{venueName}} (items: {{itemCount}}).</p>
+    <p>Please settle your tab at the bar at your earliest convenience.</p>
+    <p>Thank you!</p>
+  </body>
+</html>

apps/api/templates/emails/payment-reminder.txt

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+Hi {{customerName}},
+
+This is a friendly reminder that you have an open tab of {{amount}} at {{venueName}} (items: {{itemCount}}).
+
+Please settle your tab at the bar at your earliest convenience.
+
+Thank you!

apps/api/templates/emails/test-email.html

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+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>Test email from {{venueName}}</title>
+  </head>
+  <body>
+    <p>Hi {{customerName}},</p>
+    <p>This is a test email from {{venueName}} to verify that email reminders are working correctly.</p>
+    <p>If you received this, the email configuration is working!</p>
+    <hr>
+    <p style="color: #888; font-size: 12px;">This is a test — no action needed.</p>
+  </body>
+</html>

apps/api/templates/emails/test-email.txt

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+Hi {{customerName}},
+
+This is a test email from {{venueName}} to verify that email reminders are working correctly.
+
+If you received this, the email configuration is working!
+
+--
+This is a test — no action needed.