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/providers/base.py

@@ -17,10 +17,10 @@ class ProviderError(Exception):
 
 class Provider(ABC):
     # The endpoint's own token accounting for the most recent call, when it
-    # reported any — notably `prompt_tokens_details.cached_tokens`, which is
-    # the only direct read on whether the prompt prefix is being cached.
-    # Callers read it after the call they made; one provider is built per
-    # request, so there is nothing to race.
+    # reported any. It notably carries `prompt_tokens_details.cached_tokens`,
+    # which is the only direct measure of whether the prompt prefix is being
+    # cached. A caller reads it after the call it made, and one provider is built
+    # per request, so nothing races.
     last_usage: dict | None = None
 
     @abstractmethod