sec(api/auth): return opaque 401 in login catch-all instead of err.Error() (closes #937)

[cristim]

Summary

Fixes #937. Salvaged from closed #886.

• The login() catch-all error path was forwarding raw service errors verbatim via NewClientError(401, err.Error()), potentially exposing internal account state (lock status, user IDs, timestamps) to unauthenticated callers.
• Replaced with the opaque hardcoded string "invalid credentials" so all non-sentinel auth failures (wrong password, account not found, locked, etc.) collapse to the same opaque 401 response.
• The typed-sentinel arms above (ErrMFARequired -> "mfa_required", ErrInvalidMFACode -> "invalid_mfa_code") are unchanged.

Test plan

• TestHandler_login_OpaqueError_HidesInternalMessage (new): service returns "internal: user 42 locked since ...", response body is exactly "invalid credentials", no internal substring present.
• TestHandler_login_AuthError (existing): still passes - opaque message "invalid credentials" satisfies the existing Contains assertion.
• All other TestHandler_login_* tests unchanged and passing.
• go test ./internal/api/... -count=1 passes (1404 tests).
• gofmt -l and go vet ./internal/api/... clean.

[cristim]

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ℹ️ Review info

⚙️ Run configuration

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Run ID: 8d64b5dc-dadf-46b6-82fa-9be0e60a911e

📥 Commits

Reviewing files that changed from the base of the PR and between c00445c and 098f230.

📒 Files selected for processing (2)

• internal/api/handler_auth.go
• internal/api/handler_auth_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch sec/937-login-opaque-401

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