Add auto-update for missing configuration settings

Implemented automatic config file updates to add missing settings
with default values on startup. This eliminates manual config
editing when upgrading to newer versions.

Features:
- Auto-updates config.json with missing settings
- Auto-updates user_config.json with missing user preferences
- Console logging shows which settings were added
- Preserves existing values - only adds missing keys
- Writes updated config back to file automatically
- Graceful error handling if write fails

System config (config.json):
- Comprehensive defaults for all sections
- GPIO, audio, paths, backup, web, system settings
- New settings like volume_greeting, volume_button, volume_beep

User config (user_config.json):
- Loop-based checking against default_config
- Automatic save when updates are detected
- Error logging for troubleshooting

Benefits:
- Seamless upgrades without manual config edits
- No breaking changes when new features are added
- Users see exactly what was added in console
- Backward compatible with old config files

Example console output:
  [CONFIG] Added missing setting: system.volume_beep = 70
  [CONFIG] Updated config.json with missing defaults
  [USER_CONFIG] Added missing setting: volume_button = 70
  [USER_CONFIG] Updated user_config.json with missing defaults

Updated README.md to document auto-update feature.

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

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

README.md

@@ -491,6 +491,18 @@ rotary_phone_data/            # Default location (configurable)
 
 All configuration is done via the `config.json` file. **No need to edit Python code!**
 
+### Auto-Update Feature
+
+The system automatically adds missing configuration settings with default values when it starts:
+
+- **config.json** - System configuration is auto-updated with any missing settings
+- **user_config.json** - User settings are auto-updated with new defaults
+- Console shows which settings were added: `[CONFIG] Added missing setting: system.volume_beep = 70`
+- Original config is preserved - only missing keys are added
+- No manual editing required when upgrading to newer versions
+
+This means you can upgrade the code and your old config files will work automatically!
+
 ### Configuration File Structure
 
 The `config.json` file contains all system settings: