Switch from SQLAlchemy classical mapping to declarative

[jdavcs]

This is related to #10892, but due to scope should be easier to track as a separate issue.

Reasoning: declarative mapping style has been the recommended approach since SQLAlchemy v. 05 (01/2010):

Declarative is recommended for applications which don’t require (and don’t prefer) abstraction between tables and mappers - The [/docs/05/reference/ext/declarative.html Declarative] module, which is used to combine the expression of Table, mapper(), and user defined class objects together, is highly recommended as it simplifies application configuration, ensures the “one mapper per class” pattern, and allows the full range of configuration available to distinct mapper() calls.

Ref: https://docs.sqlalchemy.org/en/12/changelog/migration_05.html#object-relational-mapping