fix: resolve 8 SonarQube reliability bugs

- ElkDiagramRenderer: guard against null containingNode before getElkRoot()
- OpenSearchAdminController: return 503/502 instead of 200 on errors
- DatabaseAdminController: return 503 instead of 200 on connection failure
- SpaForwardController: replace unbound {path} variables with /** wildcards
- WriteBuffer: check offer() return value and log on unexpected rejection
- ApiExceptionHandler: extract getReason() to local var for null safety
- Admin UI pages: handle isError state for disconnected service display

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/diagram/ElkDiagramRenderer.java

@@ -415,12 +415,13 @@ public class ElkDiagramRenderer implements DiagramRenderer {
         for (ElkEdge elkEdge : allEdges) {
             String sourceId = elkEdge.getSources().isEmpty() ? "" : elkEdge.getSources().get(0).getIdentifier();
             String targetId = elkEdge.getTargets().isEmpty() ? "" : elkEdge.getTargets().get(0).getIdentifier();
-            ElkNode edgeRoot = getElkRoot(elkEdge.getContainingNode());
+            ElkNode containingNode = elkEdge.getContainingNode();
+            ElkNode edgeRoot = containingNode != null ? getElkRoot(containingNode) : null;
 
             List<double[]> points = new ArrayList<>();
             for (ElkEdgeSection section : elkEdge.getSections()) {
-                double cx = getAbsoluteX(elkEdge.getContainingNode(), edgeRoot);
-                double cy = getAbsoluteY(elkEdge.getContainingNode(), edgeRoot);
+                double cx = containingNode != null ? getAbsoluteX(containingNode, edgeRoot) : 0;
+                double cy = containingNode != null ? getAbsoluteY(containingNode, edgeRoot) : 0;
                 points.add(new double[]{section.getStartX() + cx, section.getStartY() + cy});
                 for (ElkBendPoint bp : section.getBendPoints()) {
                     points.add(new double[]{bp.getX() + cx, bp.getY() + cy});