Refactor Docker setup and migrate to PostgreSQL

Streamlined Dockerfiles with clearer ENV variables and build args. Switched backend database from MySQL to PostgreSQL, updated configurations accordingly, and added robust Docker Compose services for better orchestration, including health checks and persistent storage.

backend/app/database.py

@@ -26,7 +26,9 @@ else:  # Default is dev
     db_host = os.getenv("DB_HOST", "localhost")
     db_name = os.getenv("DB_NAME", "aare_dev_db")
 
-SQLALCHEMY_DATABASE_URL = f"mysql://{db_username}:{db_password}@{db_host}/{db_name}"
+SQLALCHEMY_DATABASE_URL = (
+    f"postgresql://{db_username}:{db_password}@{db_host}/{db_name}"
+)
 
 # Create engine and session
 engine = create_engine(SQLALCHEMY_DATABASE_URL)