Add user accounts to my stack

I'm building <one sentence> with <stack> on <host>, and it now needs user
accounts.

First, recommend how: this stack's built-in or standard auth library, or
a hosted auth service. Pick one for my situation and justify it in three
lines. Don't hand-roll password handling.

Then the smallest working version: sign up, log in, log out, password
reset, and a single check for "is this person logged in". Include the
session and cookie settings that matter on this host, especially over
HTTPS.