the roadmap for adapt to modern tech for backend

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### 1. Is this request related to a challenge you're experiencing? Tell me about your story.

flask is painful, it missing type checker.

the roadmap:

1. remove flask-sqlalchemy (doing) https://github.com/langgenius/dify/issues/24138
2. remove reqparse -> BaseModel (doing) https://github.com/langgenius/dify/issues/27513
3. remove api.model -> BaseModel (doing) https://github.com/langgenius/dify/pull/30388
4. ~~maybe change to https://luolingchun.github.io/flask-openapi3/latest/Quickstart/ since it's less hard, but also need to remove model~~
5. ~~adapt fastopenapi https://github.com/mr-fatalyst/fastopenapi (after 2 and 3) , so all the backend api endpoint style is fastapi-style~~
6. ~~flask -> quart https://quart.palletsprojects.com/en/latest/how_to_guides/flask_migration/ (so move to async api) , this need to deal with all the flask plugin, may take long time. and may be optional. draft ~~https://github.com/langgenius/dify/pull/31784
7. quart -> fastapi (currently final goal)

just use fastapi, use WSGI to mount flask.

### 2. Additional context or comments

this will take some time...

### 3. Can you help us with this feature?

- [x] I am interested in contributing to this feature.