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/scripting/engine.py

@@ -94,8 +94,9 @@ def run_hook(
     story_cards: list[dict],
     info: dict,
 ) -> HookResult:
-    """Run one modifier hook. Never raises: failures come back as .error with
-    text/state/cards unchanged, so a bad script can't break a turn."""
+    """Run one modifier hook. This function never raises. Failures return as
+    `.error` with text, state, and cards unchanged, so a bad script cannot
+    break a turn."""
     unchanged = HookResult(text=text, state=state, story_cards=story_cards)
     source = f"{library_js}\n;\n{hook_js}" if library_js.strip() else hook_js
     if not source.strip():