Refactor and remove sqlite and replace with MySQL

internal/storage/mysql/messages/create.go

@@ -0,0 +1,13 @@
+package storage
+
+import (
+	"database/sql"
+)
+
+func SendMessage(db *sql.DB, senderID, recipientID int, subject, message string) error {
+	_, err := db.Exec(`
+		INSERT INTO users_messages (senderId, recipientId, subject, message)
+		VALUES (?, ?, ?, ?)
+	`, senderID, recipientID, subject, message)
+	return err
+}