fix: configure stdio-based MCP servers (penpot, imap)

- Set penpot-mcp restart: "no" for on-demand execution
- Create custom Dockerfile for imap-mcp with python module entrypoint
- Fix IMAP_HOST environment variable naming (was IMAP_SERVER)
- Update .env and docker-compose.yml for IMAP_HOST
- Remove command overrides from imap-mcp and penpot-mcp
- Verify penpot-mcp and imap-mcp work correctly

.env

@@ -43,7 +43,7 @@ DISCOURSE_API_KEY=dummy-api-key-replace-with-real
 DISCOURSE_API_USERNAME=admin
 
 # IMAP
-IMAP_SERVER=imap.example.com
+IMAP_HOST=imap.example.com
 IMAP_PORT=993
 IMAP_USERNAME=user@example.com
 IMAP_PASSWORD=dummy-password-replace-with-real