fix: BASE_URL typo + nginx timeout für Workflows

Frontend:
- api.js Zeile 499: BASE_URL → BASE (executePromptStreaming)
- nginx.conf: proxy_read_timeout 300s für lange Workflow-Ausführungen

Fixes:
- "BASE_URL is not defined" Fehler in Analyse-Seite
- 504 Gateway Timeout bei Workflow-Ausführung (>60s)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/nginx.conf

@@ -8,6 +8,9 @@ server {
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
         client_max_body_size 20M;
+        proxy_read_timeout 300s;
+        proxy_connect_timeout 60s;
+        proxy_send_timeout 60s;
     }
 
     location / {

frontend/src/utils/api.js

@@ -496,7 +496,7 @@ export const api = {
 
     // Return a Promise that resolves with final result
     return new Promise((resolve, reject) => {
-      const url = `${BASE_URL}/prompts/execute-stream?${params}`
+      const url = `${BASE}/prompts/execute-stream?${params}`
 
       const eventSource = new EventSource(url)