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/web/SpaForwardController.java

@@ -16,9 +16,9 @@ public class SpaForwardController {
     @GetMapping(value = {
             "/login",
             "/executions",
-            "/executions/{path:[^\\.]*}",
+            "/executions/**",
             "/oidc/callback",
-            "/admin/{path:[^\\.]*}"
+            "/admin/**"
     })
     public String forward() {
         return "forward:/index.html";