fix(security): externalize JWT secret via environment variable

- jwt.secret now reads from JWT_SECRET env var
- Falls back to dev default if not set

eeg_backend/src/main/resources/application.yml

@@ -11,7 +11,7 @@ spring:
       ddl-auto: update
     show-sql: true
 jwt:
-  secret: MeinSuperGeheimesUndSehrLangesSecretFuerDasEnergiePortal2025!
+  secret: ${JWT_SECRET:MeinSuperGeheimesUndSehrLangesSecretFuerDasEnergiePortal2025!}
 app:
   frontend-url: http://localhost:4200
   backend-url: http://localhost:8080