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>

README.md

@@ -12,7 +12,9 @@ A Raspberry Pi-based rotary phone system for weddings and events. Guests can pic
 - **Volume Control**: Adjust playback volume with real-time slider (0-100%)
 - **Multiple Message Support**: Upload and manage multiple greeting messages
 - **Active Message Selector**: Choose which greeting plays when the phone is picked up
-- **Extra Button Support**: Optional GPIO button to play custom sounds on demand
+- **Extra Button Support**: Optional GPIO button to play custom sounds during recording
+- **USB Backup**: Automatic backup to multiple USB drives with CRC32 verification
+- **Data Integrity**: Every file write is verified with CRC checksums
 - **HiFiBerry Support**: Optimized for HiFiBerry DAC+ADC Pro audio quality
 - **Real-time Status**: Monitor phone status (on-hook/off-hook/recording)
 - **Auto-refresh**: Status updates every 5 seconds
@@ -206,6 +208,73 @@ Configure a delay before the greeting plays:
 - Example: 2 second delay gives time to position the phone
 - Default: 0 (greeting plays immediately)
 
+### USB Backup (Optional)
+
+Automatically backup all recordings and greeting files to USB drives:
+
+**Configuration** (`config.json`):
+```json
+{
+  "backup": {
+    "enabled": true,
+    "usb_paths": [
+      "/media/usb0",
+      "/media/usb1"
+    ],
+    "verify_crc": true,
+    "backup_on_write": true
+  }
+}
+```
+
+**Features:**
+- **Multiple USB Drives**: Backup to one or more USB drives simultaneously
+- **CRC32 Verification**: Every backup is verified with CRC checksum
+- **Automatic Backup**: Files are backed up immediately after recording/upload
+- **Integrity Check**: Corrupted backups are automatically deleted
+- **Web Monitoring**: View USB drive status, free space, and test backups
+
+**Web Interface:**
+- Monitor USB drive status (mounted, writable, free space)
+- Test backup functionality with one click
+- View real-time backup results
+- Green = Ready, Yellow = Not Writable, Red = Not Mounted
+
+**Backup Structure:**
+```
+/media/usb0/
+└── wedding-phone-backup/
+    ├── recordings/
+    │   ├── recording_20250124_143022.wav
+    │   └── recording_20250124_143145.wav
+    └── sounds/
+        ├── dialtone.wav
+        └── greeting.wav
+```
+
+**How It Works:**
+1. Recording finishes or greeting uploaded
+2. File saved to main storage
+3. CRC32 checksum calculated for source file
+4. File copied to each USB drive
+5. CRC32 checksum verified on each copy
+6. Corrupted copies deleted automatically
+7. Success/failure logged to console
+
+**Mount USB Drives:**
+```bash
+# Create mount points
+sudo mkdir -p /media/usb0 /media/usb1
+
+# Auto-mount in /etc/fstab (example)
+UUID=XXXX-XXXX  /media/usb0  vfat  defaults,nofail  0  0
+UUID=YYYY-YYYY  /media/usb1  vfat  defaults,nofail  0  0
+
+# Or mount manually
+sudo mount /dev/sda1 /media/usb0
+sudo mount /dev/sdb1 /media/usb1
+```
+
 ## File Structure
 
 ```
@@ -263,6 +332,12 @@ The `config.json` file contains all system settings:
     "recordings_dir": "recordings",        // Subdirectory for recordings
     "sounds_dir": "sounds"                 // Subdirectory for greeting sounds
   },
+  "backup": {
+    "enabled": true,                       // Enable USB backup
+    "usb_paths": ["/media/usb0", "/media/usb1"], // USB mount points
+    "verify_crc": true,                    // Verify backups with CRC32
+    "backup_on_write": true                // Backup immediately after write
+  },
   "web": {
     "port": 8080,                          // Web interface port
     "max_upload_size_mb": 50               // Max upload file size