fix: add tmpfs mount to Filestash for video transcoding

Video transcoding was failing with 'Failed to open segment pipe:out000.ts'
because ffmpeg couldn't create named pipes. Added tmpfs mount with exec
permissions to /tmp to allow ffmpeg to create temporary files and pipes.

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

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

stash/compose.yaml

@@ -8,6 +8,8 @@ services:
     restart: unless-stopped
     volumes:
       - filestash_data:/app/data/state/
+    tmpfs:
+      - /tmp:exec
     environment:
       TZ: ${TIMEZONE:-Europe/Berlin}
       APPLICATION_URL: ${STASH_TRAEFIK_HOST}