housekeep(fastapi): rework aurweb.db session API

Changes:
-------
- Add aurweb.db.get_session()
    - Returns aurweb.db's global `session` instance
    - Provides us a way to change the implementation of the session
      instance without interrupting user code.
- Use aurweb.db.get_session() in session API methods
- Add docstrings to session API methods
- Refactor aurweb.db.delete
    - Normalize aurweb.db.delete to an alias of session.delete
- Refresh instances in places we depend on their non-PK columns
  being up to date.

Signed-off-by: Kevin Morris <kevr@0cost.org>

test/test_db.py

@@ -279,13 +279,13 @@ def test_connection_execute_paramstyle_unsupported():
 
 def test_create_delete():
     with db.begin():
-        db.create(AccountType, AccountType="test")
+        account_type = db.create(AccountType, AccountType="test")
 
     record = db.query(AccountType, AccountType.AccountType == "test").first()
     assert record is not None
 
     with db.begin():
-        db.delete(AccountType, AccountType.AccountType == "test")
+        db.delete(account_type)
 
     record = db.query(AccountType, AccountType.AccountType == "test").first()
     assert record is None
@@ -306,7 +306,7 @@ def test_add_commit():
 
     # Remove the record.
     with db.begin():
-        db.delete(AccountType, AccountType.ID == account_type.ID)
+        db.delete(account_type)
 
 
 def test_connection_executor_mysql_paramstyle():