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/config.py

@@ -6,7 +6,6 @@ Validates required settings on startup and provides sensible defaults.
 """
 
 import os
-from typing import Optional
 
 from dotenv import load_dotenv
 
@@ -32,8 +31,8 @@ class Config:
     # SMTP Configuration
     SMTP_HOST: str
     SMTP_PORT: int
-    SMTP_USERNAME: Optional[str]
-    SMTP_PASSWORD: Optional[str]
+    SMTP_USERNAME: str | None
+    SMTP_PASSWORD: str | None
     SMTP_FROM: str
     SMTP_USE_TLS: bool