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/email.py

@@ -9,7 +9,6 @@ import logging
 import smtplib
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
-from typing import Optional
 
 logger = logging.getLogger("gondulf.email")
 
@@ -32,8 +31,8 @@ class EmailService:
         smtp_host: str,
         smtp_port: int,
         smtp_from: str,
-        smtp_username: Optional[str] = None,
-        smtp_password: Optional[str] = None,
+        smtp_username: str | None = None,
+        smtp_password: str | None = None,
         smtp_use_tls: bool = True,
     ):
         """