refactor: migrate from SQLite to PostgreSQL for session management and data storage

- Updated package dependencies to replace SQLite with PostgreSQL.
- Modified privacy policy to reflect the change in database technology.
- Adjusted session management in server.js to use connect-pg-simple.
- Enhanced docker-compose.yml to include PostgreSQL service with health checks.
- Added Dockerfile for multi-stage builds for development and production environments.
- Introduced .dockerignore to exclude unnecessary files from the Docker context.

app/events/voiceStateUpdate.js

@@ -238,8 +238,9 @@ async function trackVoiceTime(guildId, userId, oldState, newState) {
   if (newState.channelId && !oldState.channelId) {
     // Save join timestamp
     db.run(
-      `INSERT OR REPLACE INTO voice_sessions (guild_id, user_id, join_timestamp)
-       VALUES (?, ?, ?)`,
+      `INSERT INTO voice_sessions (guild_id, user_id, join_timestamp)
+       VALUES (?, ?, ?)
+       ON CONFLICT (guild_id, user_id) DO UPDATE SET join_timestamp = EXCLUDED.join_timestamp`,
       [guildId, userId, Date.now()]
     );
   }