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/main.py

@@ -106,12 +106,12 @@ class SecurityHeadersMiddleware:
 class ApiErrorMiddleware:
     """Counts failed API responses for the analytics dashboard.
 
-    Here rather than in an exception handler because it sees what the client
-    actually got: a 429 from a rate limiter, a 404 from routing, a 500 from a
-    handler that never returned, all the same way. Pure ASGI for the same
-    reason as the headers above — an SSE turn must not be buffered on its way
-    out. Only /api is watched; a 404 on the SPA mount is a page load, not a
-    fault.
+    This is middleware rather than an exception handler, because it observes
+    what the client received. A 429 from a rate limiter, a 404 from routing, and
+    a 500 from a handler that never returned all reach it the same way. It is
+    pure ASGI for the same reason as the headers above: an SSE turn must not be
+    buffered on its way out. It watches `/api` only, because a 404 on the SPA
+    mount is a page load rather than a fault.
     """
 
     def __init__(self, app):