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-core/src/main/java/com/cameleer3/server/core/ingestion/WriteBuffer.java

@@ -1,5 +1,8 @@
 package com.cameleer3.server.core.ingestion;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.ArrayBlockingQueue;
@@ -16,6 +19,8 @@ import java.util.concurrent.BlockingQueue;
  */
 public class WriteBuffer<T> {
 
+    private static final Logger log = LoggerFactory.getLogger(WriteBuffer.class);
+
     private final BlockingQueue<T> queue;
     private final int capacity;
 
@@ -45,7 +50,10 @@ public class WriteBuffer<T> {
             return false;
         }
         for (T item : items) {
-            queue.offer(item);
+            if (!queue.offer(item)) {
+                log.warn("WriteBuffer offer rejected despite capacity check — possible concurrent modification");
+                return false;
+            }
         }
         return true;
     }