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/debuglog.py

@@ -54,10 +54,13 @@ def finish_entry(
     error: str | None = None,
     usage: dict | None = None,
 ) -> None:
-    """`usage` is the endpoint's own token accounting when it reported any.
-    On OpenRouter it carries `prompt_tokens_details.cached_tokens`, which is
-    the only direct read on whether the prompt prefix is actually being
-    cached — a number worth seeing beside the request that produced it."""
+    """Finishes a log entry.
+
+    `usage` is the endpoint's own token accounting, when it reported any. On
+    OpenRouter it carries `prompt_tokens_details.cached_tokens`, which is the
+    only direct measure of whether the prompt prefix is being cached, and it is
+    worth seeing next to the request that produced it.
+    """
     entry["response"] = _clip(response)
     entry["usage"] = usage
     entry["error"] = error