feat: add ffmpeg to Filestash for video transcoding support

Added custom Dockerfile to build Filestash with ffmpeg and ffprobe installed,
enabling the built-in video transcoding plugin for seamless video playback.

Changes:
- Created stash/Dockerfile extending machines/filestash:latest
- Installed ffmpeg package with apt-get
- Updated stash/compose.yaml to build from Dockerfile
- Video transcoding plugin will automatically detect ffmpeg presence

This enables Filestash to transcode mov, mkv, avi, mpeg, and other video
formats for in-browser playback without manual conversion.

Note: Enable the video transcoding feature in Filestash admin panel at
https://stash.pivoine.art/admin after deployment.

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

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

stash/compose.yaml

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