[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>

aurweb/routers/html.py

@@ -44,8 +44,6 @@ async def language(request: Request,
     setting the language on any page, we want to preserve query
     parameters across the redirect.
     """
-    from aurweb.db import session
-
     if next[0] != '/':
         return HTMLResponse(b"Invalid 'next' parameter.", status_code=400)
 
@@ -53,8 +51,8 @@ async def language(request: Request,
 
     # If the user is authenticated, update the user's LangPreference.
     if request.user.is_authenticated():
-        request.user.LangPreference = set_lang
-        session.commit()
+        with db.begin():
+            request.user.LangPreference = set_lang
 
     # In any case, set the response's AURLANG cookie that never expires.
     response = RedirectResponse(url=f"{next}{query_string}",