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/templates/error.html

@@ -0,0 +1,19 @@
+{% extends "base.html" %}
+
+{% block title %}Error - Gondulf{% endblock %}
+
+{% block content %}
+<h1>Error</h1>
+
+<p class="error">{{ error }}</p>
+
+{% if error_code %}
+<p>Error code: <code>{{ error_code }}</code></p>
+{% endif %}
+
+{% if details %}
+<p>{{ details }}</p>
+{% endif %}
+
+<p><a href="/">Return to home</a></p>
+{% endblock %}