Replace GORM with bbolt for database operations

Migrated from GORM to bbolt for lightweight key-value storage, better aligning with project requirements. Updated repository methods to utilize bbolt's native APIs and removed dependencies associated with GORM.

db/db.go

@@ -0,0 +1,7 @@
+package db
+
+import "errors"
+
+var (
+	ErrRecordNotFound = errors.New("record not found")
+)