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>

README.md

@@ -300,7 +300,8 @@ The web interface provides four main sections:
 
 #### 4. Recordings
 - **Play**: Listen to recordings directly in the browser
-- **Download**: Save recordings to your computer
+- **Download**: Save individual recordings to your computer
+- **Download All**: Download all recordings as a single ZIP file with timestamp
 - **Delete**: Remove unwanted recordings
 - **Statistics**: View total recordings, storage used, and total duration
 
@@ -619,6 +620,7 @@ The system provides REST API endpoints:
 - `POST /delete_greeting/<filename>` - Delete greeting
 - `GET /play_audio/<type>/<filename>` - Stream audio file
 - `GET /download/<filename>` - Download recording
+- `GET /download_all` - Download all recordings as ZIP
 - `POST /delete/<filename>` - Delete recording
 - `POST /restore_default_sound` - Generate default dial tone
 - `GET /api/backup/status` - Get USB backup drive status