[Feat] user_monthly_stat 테이블 생성을 위한 마이그레이션 파일 추가

src/main/resources/db/migration/V2__create_user_monthly_stat_table.sql

@@ -0,0 +1,10 @@
+CREATE TABLE user_monthly_stat (
+    id BIGSERIAL PRIMARY KEY,
+    user_id BIGINT NOT NULL,
+    year INT NOT NULL,
+    month INT NOT NULL,
+    record_count INT NOT NULL DEFAULT 0,
+    CONSTRAINT uq_user_monthly_stat UNIQUE (user_id, year, month),
+    CONSTRAINT fk_user_monthly_stat_user FOREIGN KEY (user_id)
+        REFERENCES users(id)
+);