[FEATURE] Add social login (Google & GitHub) with dj-rest-auth + allauth

[emidiovaleretto]

Context

The API currently supports traditional authentication (username/email/password
with JWT), implemented in #3 . This issue adds social login as
an additional authentication method, not a replacement.

Goal

Let users sign in via Google or GitHub, converging into the same JWT flow that's
already in place — so the rest of the API stays unchanged regardless of how the
user authenticated.

Scope

• Install and configure dj-rest-auth + allauth
• Set up Google OAuth provider
• Set up GitHub OAuth provider
• Ensure social login returns a JWT (same as traditional flow)

Out of scope

• Frontend integration (handled separately once endpoints are ready)
• Additional providers beyond Google/GitHub

Notes

Builds on top of the JWT authentication from #3 .
Both auth paths should converge on the same JWT, so authorization across
the API remains identical.