Add USB backup system with CRC32 verification

**USB Backup Features:**
- Automatic backup to multiple USB drives
- CRC32 checksum verification for data integrity
- Configurable backup paths in config.json
- Backup on write (recordings and greeting uploads)
- Corrupted backups automatically deleted
- Web interface for monitoring and testing

**Configuration:**
- Added backup section to config.example.json
- enabled: Enable/disable USB backup
- usb_paths: Array of USB mount points
- verify_crc: Enable CRC32 verification
- backup_on_write: Backup immediately after file write

**CRC32 Implementation:**
- calculate_crc32(): Compute file checksum
- 64KB chunk reading for memory efficiency
- Source and destination file verification
- Automatic cleanup of failed copies

**Backup Functions:**
- backup_file_to_usb(): Backup with verification
- get_usb_backup_status(): Check drive status
- Mount detection, write test, free space check
- Preserves directory structure on USB

**Web Interface:**
- USB Backup card with drive status display
- Green/Yellow/Red status indicators
- Free space monitoring
- Test backup button
- Real-time status refresh
- Detailed error reporting

**Integration:**
- Recordings backed up after save
- Greetings backed up after upload
- Backup results logged to console
- Non-blocking backup execution

**API Endpoints:**
- GET /api/backup/status - Drive status
- POST /api/backup/test - Test backup

**Documentation:**
- Complete USB backup guide in README
- Mount instructions for USB drives
- CRC verification explanation
- Backup directory structure
- Web interface usage guide

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

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

config.example.json

@@ -19,6 +19,15 @@
     "recordings_dir": "recordings",
     "sounds_dir": "sounds"
   },
+  "backup": {
+    "enabled": true,
+    "usb_paths": [
+      "/media/usb0",
+      "/media/usb1"
+    ],
+    "verify_crc": true,
+    "backup_on_write": true
+  },
   "web": {
     "port": 8080,
     "max_upload_size_mb": 50