fix(security): require domain verification before authorization

CRITICAL SECURITY FIX: The authorization endpoint was bypassing domain
verification entirely, allowing anyone to authenticate as any domain.

Changes:
- Add domain verification check in GET /authorize before showing consent
- Add POST /authorize/verify-code endpoint for code validation
- Add verify_code.html and verification_error.html templates
- Add check_domain_verified() and store_verified_domain() functions
- Preserve OAuth parameters through verification flow

Flow for unverified domains:
1. GET /authorize -> Check DB for verified domain
2. If not verified: start 2FA (DNS + email) -> show code entry form
3. POST /authorize/verify-code -> validate code -> store verified
4. Show consent page
5. POST /authorize/consent -> issue authorization code

Verified domains skip directly to consent page.

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

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

src/gondulf/templates/verify_code.html

@@ -0,0 +1,51 @@
+{% extends "base.html" %}
+
+{% block title %}Verify Your Identity - Gondulf{% endblock %}
+
+{% block content %}
+<h1>Verify Your Identity</h1>
+
+<p>To sign in as <strong>{{ domain }}</strong>, please enter the verification code sent to <strong>{{ masked_email }}</strong>.</p>
+
+{% if error %}
+<div class="error">
+    <p>{{ error }}</p>
+</div>
+{% endif %}
+
+<form method="POST" action="/authorize/verify-code">
+    <!-- Pass through authorization parameters -->
+    <input type="hidden" name="domain" value="{{ domain }}">
+    <input type="hidden" name="client_id" value="{{ client_id }}">
+    <input type="hidden" name="redirect_uri" value="{{ redirect_uri }}">
+    <input type="hidden" name="response_type" value="{{ response_type }}">
+    <input type="hidden" name="state" value="{{ state }}">
+    <input type="hidden" name="code_challenge" value="{{ code_challenge }}">
+    <input type="hidden" name="code_challenge_method" value="{{ code_challenge_method }}">
+    <input type="hidden" name="scope" value="{{ scope }}">
+    <input type="hidden" name="me" value="{{ me }}">
+
+    <div class="form-group">
+        <label for="code">Verification Code:</label>
+        <input type="text"
+               id="code"
+               name="code"
+               placeholder="000000"
+               maxlength="6"
+               pattern="[0-9]{6}"
+               inputmode="numeric"
+               autocomplete="one-time-code"
+               required
+               autofocus>
+    </div>
+
+    <button type="submit">Verify</button>
+</form>
+
+<p class="help-text">
+    Did not receive a code? Check your spam folder.
+    <a href="/authorize?client_id={{ client_id }}&redirect_uri={{ redirect_uri }}&response_type={{ response_type }}&state={{ state }}&code_challenge={{ code_challenge }}&code_challenge_method={{ code_challenge_method }}&scope={{ scope }}&me={{ me }}">
+        Request a new code
+    </a>
+</p>
+{% endblock %}