Allow OAuth user creation without email

[Copilot]

OAuth providers may not share user email based on privacy settings. Previously, ensureOAuthUser rejected authentication attempts when email was missing.

Changes

• Schema: Removed not null constraint from UserModel.Email field
• MongoDB: Made email index sparse to allow multiple empty values while preserving uniqueness for non-empty emails
• Logic: ensureOAuthUser now creates users with only provider ID when email is unavailable

// Before: returned error if email empty
if strings.TrimSpace(info.Email) == "" {
    return nil, status.Error(codes.FailedPrecondition, "email is required")
}

// After: creates user with provider ID only
userModel := &domain.UserModel{GithubID: &info.ID}

Backward compatible - existing email-based flows unchanged.

Original prompt

allow email to be empty while ensureOAuthUser

The user has attached the following file paths as relevant context:

• .github/instructions/common.instructions.md

Created from VS Code.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.