Notify question authors via Resend when an admin posts the first answer.

Require email at registration (editable on profile), add a Resend mailer with idempotent first-answer sends, and keep answer saves independent of delivery.

templates/profile.html

@@ -25,6 +25,10 @@
       </div>
     </div>
 
+    <label for="email">Email</label>
+    <input id="email" name="email" type="email" required maxlength="254" autocomplete="email" value="{{.EmailVal}}">
+    <p class="hint">Required. We’ll email you when a plumber answers your question.</p>
+
     <label for="state">State</label>
     <select id="state" name="state">
       <option value=""{{if eq .StateVal ""}} selected{{end}}>Prefer not to say</option>