fix: allow direct first login for LinuxDo OAuth

[handsomjack0]

Summary

• allow direct account creation and login for first-time LinuxDo OAuth users when there is no compatible email conflict
• keep the existing direct-login path for already bound LinuxDo identities
• keep invite-only signup working by using an invitation-only pending flow instead of falling back to legacy email completion
• add regression tests for both direct-login and invite-only flows

Validation

• local test command:
  go test ./internal/handler -run ''TestLinuxDoOAuth|TestCompleteLinuxDoOAuthRegistration'' -count=1
• result: passed

Notes

• the currently deployed hotfix image was built from the same logic and is already running on the DO host

[github-actions]

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[handsomjack0]

I have read the CLA Document and I hereby sign the CLA