feat(api): add setup_complete system setting and status handlers

Add 'setup_complete' boolean to the system_settings seed data (defaults
to false) so fresh databases start in the unconfigured state.

Add two new handlers to SystemSettingsHandler:
- SetSetupComplete: marks setup_complete=true in the database
- GetSetupStatus: reads the current setup_complete value, returns
  {setup_complete: bool} JSON response, defaults to false if the
  setting row is missing or unparseable

database/schema/schema.sql

@@ -49,7 +49,8 @@ CREATE TABLE IF NOT EXISTS system_settings (
 INSERT INTO system_settings (setting_key, setting_value, description) VALUES
 ('scan_poll_interval_seconds', '60', 'How often to scan all libraries in minutes'),
 ('auto_scan_enabled', 'true', 'Whether auto-scanning is enabled system-wide'),
-('default_timezone', 'UTC', 'System default timezone')
+('default_timezone', 'UTC', 'System default timezone'),
+('setup_complete', 'false', 'Whether the initial setup wizard has been completed')
 ON CONFLICT (setting_key) DO NOTHING;
 
 -- Create refresh_tokens table