### What problem does this PR solve?

Fix #6334

Hello, I encountered the same problem in #6334. In the
`api/db/db_models.py`, it calls `obj.create_table()` unconditionally in
`init_database_tables`, before the `migrate_db()`. Specially for the
`permission` field of `user_canvas` table, it has `index=True`, which
causes `peewee` to issue a SQL trying to create the index when the field
does not exist (the `user_canvas` table already exists), so
`psycopg2.errors.UndefinedColumn: column "permission" does not exist`
occurred.

I've added a judgement in the code, to only call `create_table()` when
the table does not exist, delegate the migration process to
`migrate_db()`.

Then another problem occurs: the `migrate_db()` actually does nothing
because it failed on the first migration! The `playhouse` blindly issue
DDLs without things like `IF NOT EXISTS`, so it fails... even if the
exception is `pass`, the transaction is still rolled back. So I removed
the transaction in `migrate_db()` to make it work.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):