Add in-app help page with user manual (Romanian)

Adds a /help page served from the dashboard navigation, containing the full
Romanian user manual for operators: dashboard, playlist management, adding
media, removing files from playlists, player management, and the advanced
edited-media view.

Implementation notes
- Manual is pre-rendered to HTML with pandoc and shipped as a static asset
  (app/static/help/_manual_body.html) rather than parsed at runtime. This
  keeps the container free of a Markdown dependency and makes the page render
  identically regardless of what is installed.
- /help is login-protected; the sidebar table of contents is derived from the
  level-2 headings at request time.
- Screenshots are served from app/static/help/screenshots/ (32 images).
- Regenerate after editing the manual:
    documentatie/convert_help_page.sh
    docker compose up -d --build

Also adds
- backup_players_playlists.sh / restore_database.sh for DB backup and restore.
- .gitignore rules so local database backups (real production data), generated
  .docx files and duplicate screenshot copies are never committed.

app/templates/base.html

@@ -393,6 +393,10 @@
                         <img src="{{ url_for('static', filename='icons/playlist.svg') }}" alt="">
                         Playlists
                     </a>
+                    <a href="{{ url_for('main.help_page') }}">
+                        <img src="{{ url_for('static', filename='icons/help.svg') }}" alt="" onerror="this.style.display='none';">
+                        Ajutor
+                    </a>
                     <a href="{{ url_for('admin.admin_panel') }}">Admin</a>
                     <a href="{{ url_for('auth.logout') }}">Logout ({{ current_user.username }})</a>
                     <button class="dark-mode-toggle" onclick="toggleDarkMode()" title="Toggle Dark Mode">