fix(security): close High/Critical findings from the Stage 8 scan

- Bump github.com/rabbitmq/amqp091-go 1.12.0 -> 1.13.0 (3 Critical, 7 High,
  reachable via rabbitmq.Connect) and golang.org/x/{text,net,crypto},
  klauspost/compress; toolchain go1.26.8, builder image pinned to
  golang:1.26.8-alpine3.24.
- Build Content-Disposition per RFC 6266/8187 (ASCII fallback +
  filename*=UTF-8''), stripping CR/LF, quotes and backslashes (CWE-113).
- Cap request bodies with MaxBytesReader (HTTP_MAX_UPLOAD_BYTES, default
  2 GiB); multipart uploads over the limit return 413 (CWE-400).
- /readyz reports "unavailable" instead of raw backend errors (CWE-209).
- Drop deprecated middleware.RealIP; add X-Content-Type-Options: nosniff.
- docker-compose credentials come from ${VAR:-default}.
- Check the RegisterValidation error; annotate verified gosec false
  positives with #nosec and a reason.

Re-scan: govulncheck 0 reachable, trivy fs/image 0 High/Critical,
gosec High 0. go test -race passes against PostGIS.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/transport/http/dataset_group_handler.go

@@ -177,8 +177,7 @@ func (h *DatasetGroupHandler) uploadSLD(w http.ResponseWriter, r *http.Request)
 	if !ok {
 		return
 	}
-	if err := r.ParseMultipartForm(maxUploadBytes); err != nil {
-		httputil.WriteError(w, http.StatusBadRequest, "request must be multipart/form-data")
+	if !parseMultipartForm(w, r) {
 		return
 	}
 
@@ -228,7 +227,7 @@ func (h *DatasetGroupHandler) downloadSLD(w http.ResponseWriter, r *http.Request
 		filename = *group.SldFilename
 	}
 	w.Header().Set("Content-Type", contentType)
-	w.Header().Set("Content-Disposition", `attachment; filename="`+filename+`"`)
+	w.Header().Set("Content-Disposition", contentDisposition("attachment", filename))
 	if _, err := io.Copy(w, obj); err != nil {
 		// Headers are already sent; nothing useful to return to the client.
 		return