[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_rss.py

@@ -49,14 +49,13 @@ def packages(user):
     now = int(datetime.utcnow().timestamp())
 
     # Create 101 packages; we limit 100 on RSS feeds.
-    for i in range(101):
-        pkgbase = db.create(
-            PackageBase, Maintainer=user, Name=f"test-package-{i}",
-            SubmittedTS=(now + i), ModifiedTS=(now + i), autocommit=False)
-        pkg = db.create(Package, Name=pkgbase.Name, PackageBase=pkgbase,
-                        autocommit=False)
-        pkgs.append(pkg)
-    db.commit()
+    with db.begin():
+        for i in range(101):
+            pkgbase = db.create(
+                PackageBase, Maintainer=user, Name=f"test-package-{i}",
+                SubmittedTS=(now + i), ModifiedTS=(now + i))
+            pkg = db.create(Package, Name=pkgbase.Name, PackageBase=pkgbase)
+            pkgs.append(pkg)
     yield pkgs