Rewrite Python comments in Google developer documentation style (#12)

* Rewrite comments in Google developer documentation style

Rewrite the comments and docstrings across the backend core modules so they
read plainly. The previous prose was accurate but dense and figurative, which
made it slow to skim.

Applies the Google developer documentation style guide: short sentences, active
voice, present tense, American spelling, and no metaphors, idioms, or
rhetorical asides. Replaces em-dash chains with separate sentences.

backend/tests/test_scenario_refresh.py

@@ -75,12 +75,13 @@ def make_scenario(client, **kwargs):
 
 
 def edit_scenario(scenario_id, cards=None, **fields):
-    """Author-side edit, straight to the DB (the scenario API is tested elsewhere).
+    """Author-side edit, straight to the database. The scenario API is
+    tested elsewhere.
 
-    `cards` is the scenario's full card list afterwards. Cards are matched to
-    existing rows by name and edited in place, exactly as ScenarioEditor does
-    (PATCH /story-cards/{id}) — card ids are stable across authoring, which is
-    what source_ref tracking relies on.
+    `cards` is the scenario's full card list afterwards. Cards are matched
+    to existing rows by name and edited in place, exactly as ScenarioEditor
+    does (PATCH /story-cards/{id}). Card ids stay stable across authoring,
+    which is what source_ref tracking relies on.
     """
     db = SessionLocal()
     try: