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_ratelimit.py

@@ -103,7 +103,7 @@ def test_ratelimit_db(get: mock.MagicMock, getboolean: mock.MagicMock,
 
     # Delete the ApiRateLimit record.
     with db.begin():
-        db.delete(ApiRateLimit)
+        db.delete(db.query(ApiRateLimit).first())
 
     # Should be good to go again!
     assert not check_ratelimit(request)