feat: add template based email content (html + plaintext fallback)

backend/templates/email/vault_unlock.html

@@ -0,0 +1,20 @@
+{% extends 'email/base.html' %}
+
+{% block content %}
+    <p>
+        Time has a way of making things clearer.<br/>
+        Or heavier. Sometimes both.
+    </p>
+    <p>
+        You had something to say at this exact moment.<br/>
+        I kept it exactly as you left it. <br/>
+        Not a word changed. Not a word read.
+    </p>
+{% endblock %}
+
+{% block footnote %}
+    <p>
+        You're ready now. Or maybe you're still not.<br/>
+        Open it anyway. You won't regret it.
+    </p>
+{% endblock %}