fix: update identity last sign-in time on OAuth sign-ins

[pjpjq]

What changed

• Refresh the existing OAuth identity's last_sign_in_at timestamp when a known identity signs in again.
• Add a Google OAuth regression test that verifies auth.identities.last_sign_in_at moves forward on subsequent sign-ins.

Fixes #2563.

Testing

• go test ./internal/api -run 'TestExternal/TestSignupExternalGoogleUpdatesIdentityLastSignInAt' -count=1
• go test ./internal/api -run 'TestExternal/TestSignupExternalGoogle(_AuthorizationCode|DisableSignupSuccessWithPrimaryEmail|UpdatesIdentityLastSignInAt)$' -count=1
• go test ./internal/api -run TestExternal -count=1