fix: enable debug logging and improve DCL sync observability

- Fix logger bug where debug level (0) was treated as falsy
  - Change `||` to `??` in config.js to properly handle log level 0
  - Debug logs now work correctly when LOG_LEVEL=debug

- Add server startup logging
  - Log port, environment, and log level on server start
  - Helps verify configuration is loaded correctly

- Add DCL API request debug logging
  - Log full API request parameters when LOG_LEVEL=debug
  - API key is redacted (shows first/last 4 chars only)
  - Helps troubleshoot DCL sync issues

- Update CLAUDE.md documentation
  - Add Logging section with log levels and configuration
  - Document debug logging feature for DCL service
  - Add this fix to Recent Commits section

Note: .env file added locally with LOG_LEVEL=debug (not committed)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/backend/config.js

@@ -17,7 +17,7 @@ export const LOG_LEVEL = process.env.LOG_LEVEL || (isDevelopment ? 'debug' : 'in
 // ===================================================================
 
 const logLevels = { debug: 0, info: 1, warn: 2, error: 3 };
-const currentLogLevel = logLevels[LOG_LEVEL] || 1;
+const currentLogLevel = logLevels[LOG_LEVEL] ?? 1;
 
 function log(level, message, data) {
   if (logLevels[level] < currentLogLevel) return;