feat: send song requests to Hermes via webhook

- Add Hermes webhook settings form in /admin/settings (URL + HMAC secret)
- Add /admin/request/<id>/send-to-hermes action
- POST request data to Hermes with V2 HMAC signature and callback URL
- Hermes can POST the generated MusicGPT prompt back to /api/prompt/<id>
- Add test button, update README workflow, bump version to 0.9.0

templates/admin/request.html

@@ -473,8 +473,11 @@
       <div class="right">
         <!-- Generate Music Prompt -->
         <div class="section">
-          <h2>1. Generate & Save Music Prompt</h2>
+          <h2>1. Generate &amp; Save Music Prompt</h2>
           <button type="button" onclick="copyPromptForHermes()">Copy customer info for Hermes</button>
+          <form method="POST" action="{{ url_for('send_to_hermes', rid=req.id) }}" style="display:inline">
+            <button type="submit" onclick="return confirm('Send this request to Hermes to generate a MusicGPT prompt?')">🚀 Send to Hermes</button>
+          </form>
           <p class="copy-hint">Paste Hermes' Title, Style, and Lyrics directly into the fields below, then save or generate.</p>
 
           {% if req.musicgpt_error %}