feat(gui): add auth core, setup gate, and first-run operator creation

- Add migrations 007-010 for system config, operators, sessions, audit_log
- Implement argon2id password hashing via argon2-cffi
- Implement session-based authentication with database-stored tokens
- Add SetupGateMiddleware to redirect to /setup until first operator created
- Add SessionMiddleware to load session from cookie and attach operator
- Create /setup, /login, /logout, /change-password routes with CSRF protection
- Add periodic session cleanup task (hourly)
- Add audit logging for auth events
- Update systemd unit with EnvironmentFile for /etc/central/central.env
- Add comprehensive tests for auth, middleware, and audit modules

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

systemd/central-gui.service

@@ -9,6 +9,7 @@ User=central
 Group=central
 WorkingDirectory=/opt/central
 Environment=HOME=/opt/central
+EnvironmentFile=/etc/central/central.env
 ExecStart=/opt/central/.venv/bin/central-gui
 Restart=on-failure
 RestartSec=5
@@ -18,3 +19,6 @@ ProtectHome=true
 PrivateTmp=true
 StandardOutput=journal
 StandardError=journal
+
+[Install]
+WantedBy=multi-user.target