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/admin/settings.html

@@ -7,7 +7,8 @@
   <style>
     /*
       Admin settings / maintenance page.
-      Two-column layout for settings; table-styled MP3 metadata fields.
+      Two-column layout for revision limit, auto-refresh, SMTP config,
+      MP3 metadata defaults, database backup/restore, health stats, and reset.
     */
     :root{
       --bg:#0b0f19;