Add per-user logins alongside the shared password

The dashboard had one shared password and no concept of a user. This
adds real accounts: a username field on the login page, per-user
credentials, and the signed-in user recorded on the session, which is
the groundwork for per-user permissions later.

Credentials are salted pbkdf2 hashes in settings.json's `users` map.
Three deliberate choices there:

- settings.json rather than an env var, because a pbkdf2 hash contains
  '$' and docker compose interpolates that when it reads .env. It's also
  already the bind-mounted file that survives `up -d --build`.
- pbkdf2 rather than Werkzeug's default scrypt, which needs a hashlib
  built against OpenSSL — the macOS system Python has no hashlib.scrypt
  and would raise at hash time.
- load_users() re-reads the file per check, so adding a user or changing
  a password is live in every gunicorn worker with no restart. The old
  _runtime_password was per-worker and only ever half-applied.

Multi-user activates only once an account exists. Until then the
original LOGIN_PASSWORD path is used unchanged and the username field is
hidden, so deploying this can't lock the existing install out.
manage_users.py prompts for passwords via getpass — never echoed, never
a CLI argument — and only ever writes hashes; it refuses to remove the
last account, which would silently drop the dashboard back to the shared
password.

Two fixes carried along: login no longer requires the LOGIN_PASSWORD env
var specifically, so a settings.json-only password works instead of
locking you out; and the legacy comparison now uses hmac.compare_digest,
which the old docstring claimed but '==' didn't provide.

Also gitignores settings.json and data/ — they hold API keys and now
password hashes, were excluded from the deploy rsync but not from git.

Verified: legacy fallback still signs in with no accounts configured; a
9-case credential matrix (right/wrong user x right/wrong password, plus
the retired shared password); per-user change-password scoped to the
signed-in account and live without restart; and the add/passwd/remove
guards.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

static/css/login.css

@@ -54,6 +54,9 @@ input {
 }
 input:focus { border-color: var(--accent); }
 input::placeholder { color: var(--text-3); }
+/* Second field in a stacked pair (username above password) needs the gap
+   the single-field form never had. */
+label.stacked { margin-top: 1rem; }
 
 .error-msg {
   background: var(--surface-2);