fix: resolve logical errors, bugs, and security issues across codebase

- Fix is_admin() method shadowing is_admin database column in User model
- Fix check_password() crash when password_hash is None (OAuth-only users)
- Fix SystemSetting.all_settings() formatting error (missing newline)
- Fix MAIL_PORT returning string instead of int in config
- Fix AUTOMATION_TOKEN config formatting (missing newline before comment)
- Fix path traversal vulnerability in serve_user_audio using realpath validation
- Fix weak auth in process.py, replace session check with @login_required
- Fix int() crash on non-numeric priority in import_songs.py
- Add timeout to SMTP connection in email_helper.py
- Add timeouts to external API requests in metadata.py and spotify_helper.py
- Fix security tests to properly reload config module
- Fix metadata test mock data key mismatch (preview_url -> spotify_preview_url)
- Add skip decorator to integration test requiring live API credentials

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

musicround/routes/import_songs.py

@@ -85,7 +85,10 @@ def import_playlist():
             flash("No playlist ID provided for import.", "danger")
             return redirect(request.referrer or url_for('core.search'))
 
-        priority = int(request.form.get('priority', 10))
+        try:
+            priority = int(request.form.get('priority', 10))
+        except (ValueError, TypeError):
+            priority = 10
         queue = current_app.config.get('import_queue')
         if not queue:
             flash("Import queue not initialized.", "danger")