Remove legacy Auth0 code and configuration

[harry-rhesis]

Summary

Remove all legacy Auth0 code, configuration, and database columns now that native authentication is fully implemented.

Background / Context

The native authentication system has replaced Auth0. Legacy Auth0 compatibility is still present behind the AUTH_LEGACY_AUTH0_ENABLED flag. Per the migration reference, this code should be removed once all users have migrated. The /auth/demo endpoint is excluded from this issue and handled separately.

Deliverables

Backend:

• Remove auth0_id column from User model and create an Alembic migration to drop it
• Remove _legacy_auth0_callback() function from routers/auth.py
• Remove GET /auth/login legacy endpoint
• Remove auth/oauth.py module (Auth0 authlib OAuth setup)
• Remove AUTH_LEGACY_AUTH0_ENABLED env var and all related conditional logic
• Remove AUTH0_DOMAIN env var references
• Remove authlib dependency if no longer used elsewhere

Frontend:

• Remove any Auth0-related callback or redirect handling
• Clean up environment variables referencing Auth0

Infrastructure / Config:

• Remove Auth0 secrets and env vars from deployment configs

Acceptance Criteria

• No references to auth0 remain in the codebase (except migration history and demo endpoint)
• AUTH_LEGACY_AUTH0_ENABLED flag and all gated code paths are removed
• auth0_id column dropped via Alembic migration
• All auth flows continue to work without legacy code
• Existing tests pass; legacy-specific tests updated or removed