Enforce account-active status across the auth lifecycle

active? was only checked at the password step of sign-in. A user disabled
afterwards could (a) still complete the 2FA step and mint a valid session, and
(b) keep using any existing session until natural expiry, because per-request
auth only checked session expiry, not user status.

Three enforcement points:
- Mid-flow guard: verify_totp and webauthn_verify re-check active? before
  start_new_session_for, clearing the pending session and rejecting if disabled.
- Request-time guard: find_session_by_cookie now uses Session.for_active_user,
  so a session whose user is disabled no longer authenticates (authoritative,
  catches any disable path including direct DB changes).
- Immediate cleanup: User#revoke_sessions_when_deactivated destroys a user's
  sessions when status changes away from active, so access is revoked everywhere
  at once rather than on the next request.

Tests cover the mid-flow TOTP rejection, request-time rejection of an existing
session after disable, session destruction on disable, and that unrelated
updates leave sessions intact.

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

app/controllers/concerns/authentication.rb

@@ -31,7 +31,7 @@ module Authentication
   end
 
   def find_session_by_cookie
-    Session.active.find_by(id: cookies.signed[:session_id]) if cookies.signed[:session_id]
+    Session.active.for_active_user.find_by(id: cookies.signed[:session_id]) if cookies.signed[:session_id]
   end
 
   def request_authentication