Add cookie banner REST API with per-banner CORS middleware

Introduce /cookie-banner/v1/{bannerID}/config endpoint for the JS SDK.
The custom CORS middleware validates each request origin against the
specific banner being requested, preventing cross-customer leakage.

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

pkg/server/api/cookiebanner/v1/cors_middleware.go

@@ -0,0 +1,79 @@
+// 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 cookiebanner_v1
+
+import (
+	"errors"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+	"go.gearno.de/kit/log"
+	"go.probo.inc/probo/pkg/cookiebanner"
+	"go.probo.inc/probo/pkg/gid"
+)
+
+func newCORSMiddleware(logger *log.Logger, cookieBannerSvc *cookiebanner.Service) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			origin := r.Header.Get("Origin")
+			if origin == "" {
+				next.ServeHTTP(w, r)
+				return
+			}
+
+			bannerIDStr := chi.URLParam(r, "bannerID")
+			if bannerIDStr == "" {
+				http.Error(w, "forbidden", http.StatusForbidden)
+				return
+			}
+
+			bannerID, err := gid.ParseGID(bannerIDStr)
+			if err != nil {
+				http.Error(w, "forbidden", http.StatusForbidden)
+				return
+			}
+
+			banner, err := cookieBannerSvc.GetActiveCookieBanner(r.Context(), bannerID)
+			if err != nil {
+				if errors.Is(err, cookiebanner.ErrBannerNotFound) {
+					http.Error(w, "forbidden", http.StatusForbidden)
+					return
+				}
+				logger.ErrorCtx(r.Context(), "cannot load cookie banner for CORS check", log.Error(err))
+				http.Error(w, "internal server error", http.StatusInternalServerError)
+				return
+			}
+
+			canonicalOrigin := cookiebanner.CanonicalizeOrigin(origin)
+			if banner.Origin != canonicalOrigin {
+				http.Error(w, "forbidden", http.StatusForbidden)
+				return
+			}
+
+			w.Header().Set("Access-Control-Allow-Origin", origin)
+			w.Header().Set("Access-Control-Allow-Methods", "GET, POST, OPTIONS")
+			w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
+			w.Header().Set("Access-Control-Max-Age", "600")
+			w.Header().Set("Vary", "Origin")
+
+			if r.Method == http.MethodOptions {
+				w.WriteHeader(http.StatusNoContent)
+				return
+			}
+
+			next.ServeHTTP(w, r)
+		})
+	}
+}

pkg/server/api/cookiebanner/v1/handler.go

@@ -0,0 +1,49 @@
+// 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 cookiebanner_v1
+
+import (
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+	"go.gearno.de/kit/httpserver"
+	"go.gearno.de/kit/log"
+	"go.probo.inc/probo/pkg/cookiebanner"
+)
+
+type Handler struct {
+	logger          *log.Logger
+	cookieBannerSvc *cookiebanner.Service
+}
+
+func NewMux(
+	logger *log.Logger,
+	cookieBannerSvc *cookiebanner.Service,
+) *chi.Mux {
+	h := &Handler{
+		logger:          logger,
+		cookieBannerSvc: cookieBannerSvc,
+	}
+
+	r := chi.NewMux()
+	r.Use(newCORSMiddleware(logger, cookieBannerSvc))
+	r.Get("/{bannerID}/config", h.handleGetConfig)
+
+	return r
+}
+
+func (h *Handler) handleGetConfig(w http.ResponseWriter, r *http.Request) {
+	httpserver.RenderJSON(w, http.StatusOK, map[string]string{"status": "ok"})
+}