feat(dns): implement DKIM record serving and proactive key generation

- Add loadDkimRecords() method to read DKIM records from JSON files
- Integrate DKIM records into DNS server during startup
- Add initializeDkimForEmailDomains() for proactive DKIM key generation
- Ensure DKIM records are available immediately after server startup
- Update documentation with DKIM implementation status

DKIM records are now automatically loaded from .nogit/data/dns/*.dkimrecord.json
and served via DNS. Keys are generated for all configured email domains at startup.

readme.hints.md

@@ -1,5 +1,29 @@
 # Implementation Hints and Learnings
 
+## DKIM Implementation Status (2025-05-30)
+
+### Current Implementation
+1. **DKIM Key Generation**: Working - keys are generated when emails are sent
+2. **DKIM Email Signing**: Working - emails are signed with DKIM
+3. **DKIM DNS Record Serving**: Implemented - records are loaded from JSON files and served
+4. **Proactive DKIM Generation**: Implemented - keys are generated for all email domains at startup
+
+### Key Points
+- DKIM selector is hardcoded as `mta` in DKIMCreator
+- DKIM records are stored in `.nogit/data/dns/*.dkimrecord.json`
+- DKIM keys are stored in `.nogit/data/keys/{domain}-private.pem` and `{domain}-public.pem`
+- The server needs to be restarted for DKIM records to be loaded and served
+- Proactive generation ensures DKIM records are available immediately after startup
+
+### Testing
+After server restart, DKIM records can be queried:
+```bash
+dig @192.168.190.3 mta._domainkey.central.eu TXT +short
+```
+
+### Note
+The existing dcrouter instance has test domain DKIM records but not for production domains like central.eu. A restart is required to trigger the proactive DKIM generation for configured email domains.
+
 ## SmartProxy Usage
 
 ### New Route-Based Architecture (v18+)