sec(auth): stop leaking MFA enrollment via login error responses (closes #388)

[cristim]

Summary

• Add ErrMFANotConfigured sentinel to internal/auth/errors.go and wrap it in verifyPasswordAndMFA. Map it to "mfa_required" in the login handler (same as ErrMFARequired) so an attacker cannot distinguish "MFA enrolled and working" from "MFA enrolled but secret missing" via the 401 response.
• Replace the bare err.Error() fallthrough in the login handler with a fixed "invalid credentials" string so no internal error message ever reaches the client.
• Coordinates with PR refactor(auth/mfa): use sentinel errors in mapMFAServiceError #883 (sentinel errors for MFA management operations): ErrMFANotConfigured message string is identical to the pre-sentinel literal, so mapMFAServiceError's substring fallback still fires correctly until refactor(auth/mfa): use sentinel errors in mapMFAServiceError #883 lands.

Test plan

• TestLogin_FailedAuth_ResponseEquivalence positively asserts that wrong-password for a no-MFA user and wrong-password for an MFA-enrolled user both return HTTP 401 + "invalid credentials" -- identical bodies.
• TestLogin_MFANotConfigured_ReturnsMFARequired asserts ErrMFANotConfigured produces 401 + "mfa_required", not the internal error string.
• TestLogin_MFARequired_And_NotConfigured_ProduceSameResponse cross-checks ErrMFARequired and ErrMFANotConfigured produce the exact same status code and response body.
• TestLogin_WithMFA_NoSecret (auth package) updated to use errors.Is(err, ErrMFANotConfigured) -- a stronger assertion than Contains.
• Full suite: go test ./internal/auth/... ./internal/api/... -- 1857 tests pass.

[cristim]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 41ae072b-ce99-478a-a8c8-2882aeae1ea6

📥 Commits

Reviewing files that changed from the base of the PR and between 4956d66 and 904fe51.

📒 Files selected for processing (5)

• internal/api/handler_auth.go
• internal/api/handler_auth_test.go
• internal/auth/errors.go
• internal/auth/service.go
• internal/auth/service_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/388-wave21

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