refactor: switch Filestash from custom build to official image

Removed custom Dockerfile and switched to using the official
machines/filestash image from Docker Hub:
- Removed stash/Dockerfile (custom build with ffmpeg)
- Updated compose.yaml to use ${STASH_IMAGE} variable
- Defaults to machines/filestash:latest

This simplifies maintenance and ensures we use the officially
maintained image with proper updates via Watchtower.

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

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

stash/compose.yaml

@@ -1,9 +1,6 @@
 services:
   filestash:
-    build:
-      context: .
-      dockerfile: Dockerfile
-    image: filestash-ffmpeg:latest
+    image: ${STASH_IMAGE:-machines/filestash:latest}
     container_name: ${STASH_COMPOSE_PROJECT_NAME}_app
     restart: unless-stopped
     volumes: