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

Generated with [Devin](https://devin.ai)

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

apps/backend/internal/handlers/http_test.go

@@ -4,9 +4,14 @@ import (
 	"errors"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
+
+	"github.com/tdvorak/primora/apps/backend/internal/services"
 )
 
 func newTestContext(rawQuery string) (*gin.Context, *httptest.ResponseRecorder) {
@@ -104,4 +109,46 @@ func TestHandleErrorStatusMapping(t *testing.T) {
 			t.Fatalf("unexpected status: %d", recorder.Code)
 		}
 	})
+
+	t.Run("maps missing rows to not found", func(t *testing.T) {
+		t.Parallel()
+		ctx, recorder := newTestContext("")
+		handler.handleError(ctx, pgx.ErrNoRows)
+		if recorder.Code != http.StatusNotFound {
+			t.Fatalf("unexpected status: %d", recorder.Code)
+		}
+	})
+
+	t.Run("maps InputError to bad request with message", func(t *testing.T) {
+		t.Parallel()
+		ctx, recorder := newTestContext("")
+		handler.handleError(ctx, services.NewInputError("webhook url must be https"))
+		if recorder.Code != http.StatusBadRequest {
+			t.Fatalf("unexpected status: %d", recorder.Code)
+		}
+		if !strings.Contains(recorder.Body.String(), "webhook url must be https") {
+			t.Fatalf("expected reason in body, got %s", recorder.Body.String())
+		}
+	})
+
+	t.Run("maps unique violation to conflict without leaking", func(t *testing.T) {
+		t.Parallel()
+		ctx, recorder := newTestContext("")
+		handler.handleError(ctx, &pgconn.PgError{Code: "23505", Message: "duplicate key value violates unique constraint \"projects_slug_key\""})
+		if recorder.Code != http.StatusConflict {
+			t.Fatalf("unexpected status: %d", recorder.Code)
+		}
+		if strings.Contains(recorder.Body.String(), "projects_slug_key") || strings.Contains(recorder.Body.String(), "23505") {
+			t.Fatalf("raw constraint leaked: %s", recorder.Body.String())
+		}
+	})
+
+	t.Run("maps unknown pg error to internal error", func(t *testing.T) {
+		t.Parallel()
+		ctx, recorder := newTestContext("")
+		handler.handleError(ctx, &pgconn.PgError{Code: "XX000", Message: "internal"})
+		if recorder.Code != http.StatusInternalServerError {
+			t.Fatalf("unexpected status: %d", recorder.Code)
+		}
+	})
 }

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...)}
 }