Serverless auth stability & CommonJS module loader resolution

Eliminated HTTP 500 runtime crashes on email/password login, registration, and root routes caused by transitive ES Module imports.

Resolved an unhandled serverless runtime failure where transitive dependencies in firebase-admin (jwks-rsa and jose) threw ERR_REQUIRE_ESM on serverless environments during auth evaluation. Decoupled and lazy-loaded Firebase Admin modules so core routes, email/password sign-in, and registration evaluate without eager dependency overhead, while pinning jwks-rsa to a CommonJS-compatible release.