test: regression coverage for error mapping and client error extraction

- handlers: pgx.ErrNoRows→404, InputError→400 with message preserved,
  pgconn 23505→409 without constraint/SQLSTATE leak, unknown→500
- services: export NewInputError so error construction is testable
- frontend: vitest for errorMessage helper — server reason preferred,
  status text and fallbacks verified

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/services/platform_service.go

@@ -1534,6 +1534,8 @@ type InputError struct{ msg string }
 
 func (e *InputError) Error() string { return e.msg }
 
+func NewInputError(msg string) error { return &InputError{msg: msg} }
+
 func inputErrorf(format string, args ...any) error {
 	return &InputError{msg: fmt.Sprintf(format, args...)}
 }