Enable SQLite `db_type` in both app and test

[AlezHibali]

This PR aims to enable SQLite db_type in both flask app and pytest. The final code runs without bugs and passes all 16 tests.

Issue:

When running in an environment where no database type (MySQL, Postgresql, etc.) is configured, the project will fail at the database setup. For my Windows11, I did not download MySQL and thus opening my services.msc results in no service on MySQL. This will lead to Can't connect to MySQL server on '127.0.0.1' ([WinError 10061] 由于目标计算机积极拒绝，无法连接。 in English would be The error (2003) Can't connect to MySQL server on 'server' (10061) indicates that the network connection has been refused.

How do I fix:

I modified the settings.py to allow a SQLite database type and I set the default db_type to SQLite. After solving this, users could run the project without downloading any database services.

Below is the code snippet of changes: