Standardize datetime configuration on the backend

[ianliuwk1019]

Several existing behaviors related to backend table models timestamp columns were observed:

• Tables' "update_date" (model.py) is empty during programmatically INSERT. We would like to have initial values as "create_date" during insert.
• Previously we didn't notice the value inserted into timestamp column using datetime.datetime.now() (in model.py) is a fixed value. This means the timestamp value is determined during model is loaded, and all the records inserted with this datetime.datetime.now() (in model.py) after backend model is loaded will have the same values and not the moment the records are inserted. We will need to fix this.
• Most of the timestamp columns are timestamp WITH TIME ZON, but some tables have timestamp WITHOUT TIME ZON. We need to make them consistent.

Screenshots

Acceptance Criteria

• Discuss if needs to have or fix "update_date" in model.py to have value during initial insert.
• Fix timestamp columns for all tables to use Sqlalchemy func.utcnow() than datetime.datetime.now().
• Use timestamp WITH TIME ZON for all timestamp columns.
• Make sure all existing tests still pass.
• Document and close previous sqlacodegen tickets (on generating the model.py)

Additional context

• May need to discuss why we need to have value for the "update_date" during initial insert (so it is not empty when record is created), or if this is a good practice; and how to deal with current production empty "update_date" if we decide to have value in "update_date" initial during insert.

Related issue #1695 created to split off some refactoring of model.py