fix(auth): resolve sign-ins by linked identity

Summary

• Resolve OAuth sign-ins by an existing provider identity before falling back to email matching.
• Preserve the original Google-owned user when a linked GitHub account reports a different email or no email.
• Add account and auth controller regressions for linked GitHub identities with changed or missing emails.

Testing

• mise exec -- mix test test/hive/accounts_test.exs test/hive_web/controllers/auth_controller_test.exs test/hive_web/live/account_live/identities_test.exs
• mise exec -- mix test
• mise exec -- mix compile --warnings-as-errors

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