[Backend] group mutation & init data

[ChenBingWei1201]

What is this PR for?

1. add mutation resolver of group
2. init db with data

   What type of PR is it?

   [Feature]

Todos

What is the GitHub issue?

46

How should this be tested?

After opening apollo graphql playground, type pnpm initData to init db and use resolver AllGroup to check whether the data is created successfully

Screenshots (if appropriate)

Questions:

• Do the license files need updating? No
• Are there breaking changes for older versions? No
• Does this need new documentation? Yes

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