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_packages_util.py

@@ -2,6 +2,7 @@ from datetime import datetime
 
 import pytest
 
+from fastapi import HTTPException
 from fastapi.testclient import TestClient
 
 from aurweb import asgi, db
@@ -98,3 +99,8 @@ def test_query_notified(maintainer: User, package: Package):
     query = db.query(Package).filter(Package.ID == package.ID).all()
     query_notified = util.query_notified(query, maintainer)
     assert query_notified[package.PackageBase.ID]
+
+
+def test_pkgreq_by_id_not_found():
+    with pytest.raises(HTTPException):
+        util.get_pkgreq_by_id(0)