docs(08): align all .example config files with SSOT architecture

config/backend.yaml.example

@@ -111,10 +111,6 @@ application:
   # Environment: BACKEND_APPLICATION_LOGS_DIR or LOGS_DIR
   logs_dir: "./logs"
 
-  # Comma-separated list of extra allowed CORS origins (IPs/FQDNs)
-  # Environment: BACKEND_APPLICATION_CORS_ORIGINS or EXTRA_ALLOWED_ORIGINS
-  cors_origins: "http://localhost:8917"
-
 # --- Feature Flags ---
 features:
   # Enable AI image extraction and OCR?