revert: remove SFTP integration from AI stack

Removed custom Dockerfile and SFTP function integration in favor of
the simpler REST API approach (webui-export.py).

Changes:
- Restored webui service to use official Open WebUI image
- Removed custom Dockerfile.webui (paramiko build)
- Removed ai/functions/save_to_disk.py SFTP function
- Removed SSH key and functions volume mounts

The REST API export script (webui-export.py) is a simpler and more
flexible solution that doesn't require Docker modifications.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

ai/compose.yaml

@@ -24,10 +24,7 @@ services:
 
   # Open WebUI - ChatGPT-like interface for AI models
   webui:
-    build:
-      context: .
-      dockerfile: ./ai/Dockerfile.webui
-    image: ${AI_WEBUI_IMAGE:-ai_webui_custom:latest}
+    image: ${AI_WEBUI_IMAGE:-ghcr.io/open-webui/open-webui:main}
     container_name: ${AI_COMPOSE_PROJECT_NAME}_webui
     restart: unless-stopped
     environment:
@@ -66,8 +63,6 @@ services:
 
     volumes:
       - ai_webui_data:/app/backend/data
-      - ./ai/functions:/app/backend/data/functions:ro
-      - /root/.ssh/id_rsa:/app/.ssh/id_rsa:ro
     depends_on:
       - ai_postgres
       - litellm