fix: handle non-JSON responses in test email and prevent cascading failures

- Add content-type check in TestEmailButton before parsing response as JSON
- Wrap updateEmailLog in error handler with try-catch to prevent double failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/admin/TestEmailButton.tsx

@@ -126,6 +126,16 @@ export const TestEmailButton: React.FC = () => {
         }),
       })
 
+      const contentType = response.headers.get('content-type') || ''
+      if (!contentType.includes('application/json')) {
+        setResult({
+          success: false,
+          message: `Server-Fehler (HTTP ${response.status}). Bitte Seite neu laden und erneut versuchen.`,
+        })
+        setStatus('error')
+        return
+      }
+
       const data = await response.json()
 
       if (response.ok && data.success) {

src/lib/email/tenant-email-service.ts

@@ -264,10 +264,14 @@ export async function sendTenantEmail(
 
     // Log aktualisieren (status: failed)
     if (logId) {
-      await updateEmailLog(payload, logId, {
-        status: 'failed',
-        error: errorMessage,
-      })
+      try {
+        await updateEmailLog(payload, logId, {
+          status: 'failed',
+          error: errorMessage,
+        })
+      } catch (logError) {
+        console.error(`[Email] Failed to update email log ${logId}:`, logError)
+      }
     }
 
     console.error(`[Email] Error for tenant ${tenantId}:`, error)