Debug FastAPI/Flutter Login Issues

I'm seeking an experienced backend developer to debug and resolve login issues in my FastAPI-based API, which connects to a Flutter frontend. The specific problems are with email/password login and social logins (Google, Apple). We are encountering "invalid token" error messages.

Key Requirements:
- Expertise in FastAPI authentication (OAuth2, JWT)
- Knowledge of SendGrid and familiar with Firebase/Google/Apple login flows
- Ability to test and validate login endpoints end-to-end

Bonus Skills:
- Experience with AWS (RDS, Lambda) integration