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/services/dcl.service.js

@@ -58,6 +58,17 @@ export async function fetchQSOsFromDCL(dclApiKey, sinceDate = null) {
     requestBody.qsl_since = dateStr;
   }
 
+  // Debug log request parameters (redact API key)
+  logger.debug('DCL API request parameters', {
+    url: DCL_API_URL,
+    method: 'POST',
+    key: dclApiKey ? `${dclApiKey.substring(0, 4)}...${dclApiKey.substring(dclApiKey.length - 4)}` : null,
+    limit: requestBody.limit,
+    qsl_since: requestBody.qsl_since,
+    qso_since: requestBody.qso_since,
+    cnf_only: requestBody.cnf_only,
+  });
+
   try {
     const controller = new AbortController();
     const timeoutId = setTimeout(() => controller.abort(), REQUEST_TIMEOUT);