Fix multi-worker temp file issue with file-based storage

- New temp_storage.py module stores files on disk instead of in-memory
- Multiple Gunicorn workers can now share temp files
- Startup cleanup removes leftover files from previous runs
- Dockerfile creates temp_files directory
- Added temp_files/ to .gitignore

Previously encode preview worked but download failed with "File expired"
because each worker had its own in-memory TEMP_FILES dict.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -81,3 +81,6 @@ tests/
 *.img.zst
 pishrink.sh
 *.img.zst.zip
+
+# Temp file storage
+frontends/web/temp_files/