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/app/routers/story_cards.py

@@ -6,15 +6,19 @@ from ..database import get_db
 
 router = APIRouter(prefix="/api/story-cards", tags=["story-cards"])
 
-# AI Dungeon world-info / story-card array format. Its field names differ from
-# our columns: value<->entry, title<->name, description<->notes. The extra
-# `useForCharacterCreation` flag has no equivalent here — ignored on import,
-# emitted as false on export so round-tripping through AI Dungeon stays valid.
+# The AI Dungeon world-info and story-card array format. Its field names differ
+# from this app's columns: `value` maps to `entry`, `title` maps to `name`, and
+# `description` maps to `notes`. The extra `useForCharacterCreation` flag has no
+# equivalent here. It is ignored on import and written as false on export, so a
+# round trip through AI Dungeon stays valid.
 
 
 def _visible_owner(scenario_id, adventure_id, db, user):
-    """Resolve the scenario/adventure a caller may *read* cards from (public
-    demo scenarios included), or 404/422."""
+    """Resolves the scenario or adventure a caller may read cards from.
+
+    Public demo scenarios are included. The function raises a 404 or a 422 when
+    it cannot resolve one.
+    """
     if (scenario_id is None) == (adventure_id is None):
         raise HTTPException(422, "Provide exactly one of scenario_id or adventure_id")
     if scenario_id is not None: