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/routers/auth.py

@@ -53,15 +53,19 @@ def me_payload(user: models.User, db: Session) -> dict:
 
 @router.get("/me")
 def me(request: Request, response: Response, db: Session = Depends(get_db)):
-    """Who am I? In multi-user mode this also bootstraps the session: with no
-    (or an invalid) cookie it creates a guest user and sets one — the
-    frontend calls this on load and after any 401."""
+    """Returns the current user.
+
+    In multi-user mode this also establishes the session. If the cookie is
+    missing or invalid, the endpoint creates a guest user and sets a cookie. The
+    frontend calls it on load and after any 401.
+    """
     if not auth.MULTI_USER:
         user = auth.local_user(db)
     else:
         user = auth.resolve_session_user(request, db)
         if user is None:
-            # Each new guest is a database row — cap how fast one IP can mint them.
+            # Each new guest is a database row, so cap how fast one IP can
+            # create them.
             limits.rate_limit("guest", request)
             user = models.User(is_guest=True)
             db.add(user)
@@ -80,8 +84,11 @@ def register(
     db: Session = Depends(get_db),
     user: models.User = Depends(auth.get_current_user),
 ):
-    """Upgrade the current guest in place — same user_id, so every adventure,
-    scenario, script and setting they created as a guest is kept."""
+    """Upgrades the current guest in place.
+
+    The `user_id` does not change, so every adventure, scenario, script, and
+    setting they created as a guest is kept.
+    """
     if not auth.MULTI_USER:
         raise HTTPException(400, "Accounts are disabled in local mode.")
     limits.rate_limit("auth", request)