Signup

SIMPLIFIED Signup/Login endpoint.

Two main flows:

1. GitHub LINKING (linkToUserId provided):

   • Find existing user by linkToUserId (SSO UID)
   • Add GitHub provider with githubFirebaseUid as provider_uid

2. Normal GitHub sign-in (no linkToUserId):

   • Check if GitHub UID already linked to a user
   • If yes: Return that user
   • If no: Create new user with GitHub UID