make flask migration models compatible to both postgresql and mysql

[whhe]

Summary

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This PR is a merge of https://github.com/langgenius/dify/pull/15653 and https://github.com/langgenius/dify/pull/15757, I tried my best to keep the columns in models, and the table structures in postgresql should not change in general.

Resolves #15411 and resolves #15756.

Screenshots

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Checklist

[!IMPORTANT]
Please review the checklist below before submitting your pull request.

• [ ] This change requires a documentation update, included: Dify Document
• [x] I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• [ ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ ] I've updated the documentation accordingly.
• [x] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[laipz8200]

Hi there! Compatibility with MySQL isn't currently in our plans.

[FyhSky]

@whhe Using these changes, can the MySQL database be used? Have you conducted advance tests on your side?

We are planning to migrate to mysql

[whhe]

@whhe Using these changes, can the MySQL database be used? Have you conducted advance tests on your side?

We are planning to migrate to mysql

You can try on https://github.com/oceanbase/dify-on-mysql/tree/1.4.0

[whhe]

We will consider contributing at a more suitable time in the future. This PR is temporarily closed for now.