chore(deps): update @google-cloud/firestore to ^8.0.0 to resolve @tootallnate/once vulnerability

Replace internal import of DEFAULT_DATABASE_ID from @google-cloud/firestore/build/src/path
with a local constant, as v8 no longer ships .d.ts for that internal module.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: ouzkilic