Add download all recordings as ZIP feature

- Add /download_all endpoint to create in-memory ZIP archive
- Include zipfile and io imports for ZIP creation
- Add "Download All as ZIP" button in web interface (only shows when recordings exist)
- ZIP filename includes timestamp: wedding_recordings_YYYYMMDD_HHMMSS.zip
- Only includes .wav files from recordings directory
- Update API documentation in README.md
- Document feature in CLAUDE.md

Allows users to quickly backup all guest recordings in a single download.

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

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

CLAUDE.md

@@ -178,6 +178,14 @@ Edit the `RotaryPhone` class methods:
 - Backup called after recording saves and greeting uploads
 - Test with `/api/backup/test` endpoint
 
+### Download All Recordings
+The `/download_all` endpoint creates a ZIP archive of all recordings in memory:
+- Uses `zipfile` and `io.BytesIO()` for in-memory ZIP creation
+- Filename includes timestamp: `wedding_recordings_YYYYMMDD_HHMMSS.zip`
+- Only includes `.wav` files from recordings directory
+- Returns 404 if no recordings exist
+- Web UI shows "Download All as ZIP" button when recordings are present
+
 ## Testing
 
 ### Audio Testing