Extract clientip middleware and add jsonutil helpers

Move clientIP extraction into a reusable pkg/server/api/clientip
package with RFC 7239 Forwarded header support. Add
pkg/server/jsonutil with helpers for common HTTP error responses
(RenderForbidden, RenderInternalServerError, RenderNotFound,
RenderBadRequest) and use them in the cookie banner handlers.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/server/jsonutil/errors.go

@@ -0,0 +1,38 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package jsonutil
+
+import (
+	"fmt"
+	"net/http"
+
+	"go.gearno.de/kit/httpserver"
+)
+
+func RenderForbidden(w http.ResponseWriter) {
+	httpserver.RenderError(w, http.StatusForbidden, fmt.Errorf("forbidden"))
+}
+
+func RenderInternalServerError(w http.ResponseWriter) {
+	httpserver.RenderError(w, http.StatusInternalServerError, fmt.Errorf("internal server error"))
+}
+
+func RenderNotFound(w http.ResponseWriter, err error) {
+	httpserver.RenderError(w, http.StatusNotFound, err)
+}
+
+func RenderBadRequest(w http.ResponseWriter, err error) {
+	httpserver.RenderError(w, http.StatusBadRequest, err)
+}