docs: update comments and README to reflect current feature set

- Refresh app.py module and route docstrings for runtime settings,
  MP3 tagging, rate limiting, revision workflow, and admin actions.
- Clarify config.py and models.py comments.
- Update template comments/CSS for dashboard filters, request detail,
  player page, and settings page.
- Rewrite README.md with current features, status flow, file layout,
  deployment variables, and troubleshooting.
- Refresh REVIEW.md quick-reference.
- Add MAX_REVISIONS to docker-compose.yml environment list.
- Expand requirements.txt comment coverage.

No version history or changelog included.

templates/player.html

@@ -11,6 +11,7 @@
       plus approval buttons or a revision note form.
       After the customer makes a choice, the controls are hidden
       and a confirmation/waiting message is shown instead.
+      The number of remaining revisions is shown when available.
     */
     body{
       font-family:system-ui,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,sans-serif;
@@ -120,7 +121,7 @@
     </div>
 
     {% else %}
-    <!-- Approval form: customer picks A, B, or both -->
+    <!-- Approval form: customer picks Version A, B, or both -->
     <form method="POST" action="{{ url_for('approve', token=req.player_token) }}">
       <input type="hidden" name="choice" id="choice">
       <div class="actions">