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/database.py

@@ -15,9 +15,10 @@ DB_PATH = (
     else Path(__file__).resolve().parent.parent / "data.db"
 )
 
-# AIDND_DATABASE_URL (or the platform-conventional DATABASE_URL) switches the
-# app to a server database — any SQLAlchemy URL works, but Postgres is what
-# hosted deploys use (Phase 9 decision: Neon). Unset = SQLite, as always.
+# `AIDND_DATABASE_URL`, or the conventional `DATABASE_URL`, switches the app to
+# a server database. Any SQLAlchemy URL works, and hosted deploys use Postgres,
+# which Phase 9 settled on Neon for. If neither variable is set, the app uses
+# SQLite.
 DATABASE_URL = (
     os.environ.get("AIDND_DATABASE_URL", "").strip()
     or os.environ.get("DATABASE_URL", "").strip()