fix: remove global sync.Once to prevent DB state leakage in fuzz tests

internal/fuzz/db.go

@@ -1,27 +1,20 @@
 package fuzz
 
 import (
-	"sync"
-
 	"gorm.io/driver/sqlite"
 	"gorm.io/gorm"
 	"gorm.io/gorm/logger"
 )
 
-var (
-	fuzzDBOnce sync.Once
-	fuzzDB     *gorm.DB
-	fuzzDBErr  error
-)
-
 func GetFuzzDB() (*gorm.DB, error) {
-	fuzzDBOnce.Do(func() {
-		dbName := "file:memdb_fuzz?mode=memory&cache=shared&_journal_mode=WAL&_synchronous=NORMAL"
-		fuzzDB, fuzzDBErr = gorm.Open(sqlite.Open(dbName), &gorm.Config{
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{
 		Logger: logger.Default.LogMode(logger.Silent),
 	})
-		if fuzzDBErr == nil {
-			fuzzDBErr = fuzzDB.Exec(`
+	if err != nil {
+		return nil, err
+	}
+
+	err = db.Exec(`
 		CREATE TABLE IF NOT EXISTS users (
 			id INTEGER PRIMARY KEY AUTOINCREMENT,
 			username TEXT UNIQUE NOT NULL,
@@ -83,7 +76,6 @@ func GetFuzzDB() (*gorm.DB, error) {
 			FOREIGN KEY(user_id) REFERENCES users(id)
 		);
 	`).Error
-		}
-	})
-	return fuzzDB, fuzzDBErr
+
+	return db, err
 }