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/verify_email.html

@@ -0,0 +1,19 @@
+{% extends "base.html" %}
+
+{% block title %}Verify Email - Gondulf{% endblock %}
+
+{% block content %}
+<h1>Verify Your Email</h1>
+<p>A verification code has been sent to <strong>{{ masked_email }}</strong></p>
+<p>Please enter the 6-digit code to complete verification:</p>
+
+{% if error %}
+<p class="error">{{ error }}</p>
+{% endif %}
+
+<form method="POST" action="/api/verify/code">
+    <input type="hidden" name="domain" value="{{ domain }}">
+    <input type="text" name="code" placeholder="000000" maxlength="6" required autofocus>
+    <button type="submit">Verify</button>
+</form>
+{% endblock %}