Implement Google OAuth Flow

[singhaditya21]

Implemented a complete Google OAuth Authorization Code flow.
The web-demo now uses the Google Identity Services library (gsi/client) to initiate the sign-in flow and retrieve an authorization code.
The backend API exposes a new endpoint POST /api/auth/google which accepts the code, exchanges it for an access token and ID token from Google, verifies the user, and issues a JWT for the application.
Configuration was updated to include Google Client credentials.
Frontend changes were verified visually (screenshot google_login_ui.png) and backend logic was verified with unit tests.

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PR created automatically by Jules for task 2211627360713653871 started by @singhaditya21

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[singhaditya21]

Reviewed, leaving open — the feature is wanted, but three things need sorting before it can land:

1. Stray artifacts. The diff includes google_login_ui.png and verification/verify_google_ui.py. Committed screenshots and one-off verification scripts were cleared out of this repo in the recent cleanup; please drop both.
2. Needs a rebase. config.py, routers/users.py and schemas/user.py have all moved since February.
3. Cannot be verified here. The flow needs real GOOGLE_CLIENT_ID / GOOGLE_CLIENT_SECRET credentials, so I can only review it statically.

main now has a test suite with a working Postgres fixture (backend/api/tests/conftest.py, auth_client), so the token-exchange path can be tested against a stubbed Google endpoint — worth adding here.