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>

ui/src/pages/Admin/DatabaseAdminPage.tsx

@@ -3,7 +3,8 @@ import type { Column } from '@cameleer/design-system';
 import { useDatabaseStatus, useConnectionPool, useDatabaseTables, useActiveQueries, useKillQuery } from '../../api/queries/admin/database';
 
 export default function DatabaseAdminPage() {
-  const { data: status } = useDatabaseStatus();
+  const { data: status, isError: statusError } = useDatabaseStatus();
+  const unreachable = statusError || (status && !status.connected);
   const { data: pool } = useConnectionPool();
   const { data: tables } = useDatabaseTables();
   const { data: queries } = useActiveQueries();
@@ -34,7 +35,7 @@ export default function DatabaseAdminPage() {
       <h2 style={{ marginBottom: '1rem' }}>Database Administration</h2>
 
       <div style={{ display: 'flex', gap: '1rem', marginBottom: '1.5rem', flexWrap: 'wrap' }}>
-        <StatCard label="Status" value={status?.connected ? 'Connected' : 'Disconnected'} accent={status?.connected ? 'success' : 'error'} />
+        <StatCard label="Status" value={unreachable ? 'Disconnected' : status ? 'Connected' : '\u2014'} accent={unreachable ? 'error' : status ? 'success' : undefined} />
         <StatCard label="Version" value={status?.version ?? '—'} />
         <StatCard label="TimescaleDB" value={status?.timescaleDb ? 'Enabled' : 'Disabled'} />
       </div>