[FastAPI] Refactor db modifications

For SQLAlchemy to automatically understand updates from the
external world, it must use an `autocommit=True` in its session.

This change breaks how we were using commit previously, as
`autocommit=True` causes SQLAlchemy to commit when a
SessionTransaction context hits __exit__.

So, a refactoring was required of our tests: All usage of
any `db.{create,delete}` must be called **within** a
SessionTransaction context, created via new `db.begin()`.

From this point forward, we're going to require:

```
with db.begin():
    db.create(...)
    db.delete(...)
    db.session.delete(object)
```

With this, we now get external DB modifications automatically
without reloading or restarting the FastAPI server, which we
absolutely need for production.

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

test/test_group.py

@@ -2,7 +2,7 @@ import pytest
 
 from sqlalchemy.exc import IntegrityError
 
-from aurweb.db import create
+from aurweb import db
 from aurweb.models.group import Group
 from aurweb.testing import setup_test_db
 
@@ -13,13 +13,14 @@ def setup():
 
 
 def test_group_creation():
-    group = create(Group, Name="Test Group")
+    with db.begin():
+        group = db.create(Group, Name="Test Group")
     assert bool(group.ID)
     assert group.Name == "Test Group"
 
 
 def test_group_null_name_raises_exception():
-    from aurweb.db import session
     with pytest.raises(IntegrityError):
-        create(Group)
-    session.rollback()
+        with db.begin():
+            db.create(Group)
+    db.rollback()