feat: implement Story 5.1 - Magic Link Request

Allow participants to request magic links for dashboard access:

- POST /exchange/<slug>/request-access handles form submission
- Accept email, look up participant in database
- Generate token (secrets.token_urlsafe(32)), store SHA-256 hash
- Send magic link email via EmailService.send_magic_link()
- Rate limit: 3 requests per hour per email
- Always show generic success message (prevent enumeration)
- Only send email if participant exists
- Case-insensitive email lookup
- Comprehensive test suite with 7 tests

Includes:
- MagicLinkRequestForm with email validation
- request_access.html template
- GET endpoint to display request form

All tests passing (97 total), 91% coverage maintained.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/templates/participant/request_access.html

@@ -0,0 +1,38 @@
+{% extends "layouts/base.html" %}
+
+{% block title %}Request Access - {{ exchange.name }}{% endblock %}
+
+{% block content %}
+<article>
+    <header>
+        <h1>Request Access</h1>
+        <p>{{ exchange.name }}</p>
+    </header>
+
+    {% if success %}
+    <section>
+        <p><strong>Check your email!</strong></p>
+        <p>If you're registered for this exchange, we've sent you a magic link to access your participant dashboard.</p>
+        <p>The link will expire in 1 hour.</p>
+    </section>
+    {% else %}
+    <section>
+        <p>Enter your email address to receive a magic link for access to your participant dashboard.</p>
+
+        <form method="POST">
+            {{ form.hidden_tag() }}
+
+            <label for="email">
+                {{ form.email.label }}
+                {{ form.email(placeholder="your.email@example.com") }}
+                {% if form.email.errors %}
+                    <small>{{ form.email.errors[0] }}</small>
+                {% endif %}
+            </label>
+
+            <button type="submit">Send Magic Link</button>
+        </form>
+    </section>
+    {% endif %}
+</article>
+{% endblock %}