tests: gate live service-api tests behind ROOM_CAL_LIVE_TESTS=1
Tests marked 'live' hit real service APIs and need credentials dev
systems only sometimes have (especially 2FA-gated backends). They now
skip by default; set ROOM_CAL_LIVE_TESTS=1 to run them.
Marked: test_refresh_cookies (module), test_services (module — does
live price writes), test_fetch_reservations, test_get_thread_live,
test_fetch_prices_syncs_api_prices_to_caldav,
test_diff_returns_complete_structure.
Default suite drops from ~3.5m to ~7s on a machine with live creds.