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/package.json

@@ -25,12 +25,12 @@
   "dependencies": {
     "canvas": "^3.2.1",
     "chartjs-node-canvas": "^4.1.6",
-    "connect-sqlite3": "^0.9.16",
+    "connect-pg-simple": "^10.0.0",
     "cross-fetch": "^4.1.0",
     "discord.js": "^14.25.1",
     "dotenv": "^17.2.3",
     "express": "^5.2.1",
     "express-session": "^1.18.2",
-    "sqlite3": "^5.1.7"
+    "pg": "^8.16.3"
   }
 }