Installs Flask-WTF and enables CSRFProtect globally. Adds csrf_token
hidden fields to all four POST forms (login, delete entry, add user,
delete user, and the public guestbook form). Exempts the API endpoint
which uses header-based key auth instead.
Convert UTC timestamps from SQLite to Mountain Time (America/Denver)
using a Jinja2 template filter backed by zoneinfo; add tzdata dependency
for IANA timezone data in the slim Docker image.
Replaces browser-cached Basic Auth credentials with proper server-side
session management. Logout now fully invalidates the session. Adds an
HTML login form at /admin/login, SECRET_KEY env var support, and updates
README with key generation instructions and role table.