refactor(#14): FastAPI backend 라우터, 서비스, DB 모듈 분리

backend/database.py

@@ -0,0 +1,47 @@
+import os
+
+from dotenv import load_dotenv
+from psycopg_pool import ConnectionPool
+
+load_dotenv()
+
+_db_pool: ConnectionPool | None = None
+
+
+def open_database_pool() -> None:
+    """애플리케이션 시작 시 PostgreSQL connection pool을 생성합니다."""
+    global _db_pool
+
+    if _db_pool is not None:
+        return
+
+    database_url = os.getenv("DATABASE_URL")
+
+    if not database_url:
+        raise RuntimeError("DATABASE_URL is not set")
+
+    _db_pool = ConnectionPool(
+        conninfo=database_url,
+        min_size=1,
+        max_size=10,
+        open=True,
+    )
+
+
+def close_database_pool() -> None:
+    """애플리케이션 종료 시 PostgreSQL connection pool을 닫습니다."""
+    global _db_pool
+
+    if _db_pool is None:
+        return
+
+    _db_pool.close()
+    _db_pool = None
+
+
+def get_db_connection():
+    """생성된 connection pool에서 DB connection을 반환합니다."""
+    if _db_pool is None:
+        raise RuntimeError("Database connection pool is not initialized")
+
+    return _db_pool.connection()