feat(phase-2): implement domain verification system

Implements complete domain verification flow with:
- rel=me link verification service
- HTML fetching with security controls
- Rate limiting to prevent abuse
- Email validation utilities
- Authorization and verification API endpoints
- User-facing templates for authorization and verification flows

This completes Phase 2: Domain Verification as designed.

Tests:
- All Phase 2 unit tests passing
- Coverage: 85% overall
- Migration tests updated

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

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

src/gondulf/dns.py

@@ -6,7 +6,6 @@ and fallback to public DNS servers.
 """
 
 import logging
-from typing import List, Optional
 
 import dns.resolver
 from dns.exception import DNSException
@@ -51,7 +50,7 @@ class DNSService:
 
         return resolver
 
-    def get_txt_records(self, domain: str) -> List[str]:
+    def get_txt_records(self, domain: str) -> list[str]:
         """
         Query TXT records for a domain.