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/api/clientip/clientip.go

@@ -0,0 +1,106 @@
+// 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 clientip
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"strings"
+)
+
+type ctxKey struct{}
+
+// NewMiddleware returns an HTTP middleware that extracts the client IP
+// from standard proxy headers and stores it in the request context.
+func NewMiddleware() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			ip := Extract(r)
+			ctx := context.WithValue(r.Context(), ctxKey{}, ip)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}
+
+// FromContext returns the client IP stored by the middleware, or an
+// empty string if the middleware has not run.
+func FromContext(ctx context.Context) string {
+	if ip, ok := ctx.Value(ctxKey{}).(string); ok {
+		return ip
+	}
+	return ""
+}
+
+// Extract resolves the client IP address from standard proxy headers
+// in priority order: RFC 7239 Forwarded, then X-Forwarded-For, then
+// the connection's remote address.
+func Extract(r *http.Request) string {
+	if fwd := r.Header.Get("Forwarded"); fwd != "" {
+		if ip := parseForwardedFor(fwd); ip != "" {
+			return ip
+		}
+	}
+
+	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+		if i := strings.IndexByte(xff, ','); i != -1 {
+			xff = xff[:i]
+		}
+		xff = strings.TrimSpace(xff)
+
+		if ip, _, err := net.SplitHostPort(xff); err == nil {
+			return ip
+		}
+		return xff
+	}
+
+	ip, _, err := net.SplitHostPort(r.RemoteAddr)
+	if err != nil {
+		return r.RemoteAddr
+	}
+
+	return ip
+}
+
+// parseForwardedFor extracts the client IP from the first "for=" directive
+// of an RFC 7239 Forwarded header value.
+func parseForwardedFor(header string) string {
+	if i := strings.IndexByte(header, ','); i != -1 {
+		header = header[:i]
+	}
+
+	for _, part := range strings.Split(header, ";") {
+		part = strings.TrimSpace(part)
+		if !strings.HasPrefix(strings.ToLower(part), "for=") {
+			continue
+		}
+
+		val := part[4:]
+		val = strings.Trim(val, "\"")
+
+		if strings.HasPrefix(val, "[") {
+			if end := strings.IndexByte(val, ']'); end != -1 {
+				return val[1:end]
+			}
+		}
+
+		if ip, _, err := net.SplitHostPort(val); err == nil {
+			return ip
+		}
+		return val
+	}
+
+	return ""
+}