Deprecate GOOGLE_APPLICATION_CREDENTIALS

[luarss]

In favor of GEMINI_API_KEY

[harsh-kumar-patwa]

Hi, I noticed GOOGLE_APPLICATION_CREDENTIALS isn't actually used anywhere in the backend code anymore — everything has already moved to GOOGLE_API_KEY. The only references left are in .env.example, the CI workflow, and some docs. Happy to clean those up if this is still needed.