feat: implement Story 4.1 - Access Registration Page

Allows potential participants to view exchange details and access
the registration form via unique slug URLs.

Implementation:
- Added ParticipantRegistrationForm with name, email, gift_ideas, and reminder fields
- Created GET /exchange/<slug>/register route
- Built responsive registration template with exchange details
- Exempted participant routes from admin setup requirement
- Comprehensive test coverage for all scenarios

Acceptance Criteria Met:
- Valid slug displays registration form with exchange details
- Invalid slug returns 404
- Form includes all required fields with CSRF protection
- Registration deadline is displayed

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

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

src/templates/participant/register.html

@@ -0,0 +1,72 @@
+{% extends "layouts/base.html" %}
+
+{% block title %}Register for {{ exchange.name }}{% endblock %}
+
+{% block content %}
+<article>
+    <header>
+        <h1>{{ exchange.name }}</h1>
+        {% if exchange.description %}
+        <p>{{ exchange.description }}</p>
+        {% endif %}
+    </header>
+
+    <section>
+        <h2>Exchange Details</h2>
+        <dl>
+            <dt>Budget</dt>
+            <dd>{{ exchange.budget }}</dd>
+
+            <dt>Gift Exchange Date</dt>
+            <dd>{{ exchange.exchange_date.strftime('%Y-%m-%d') }}</dd>
+
+            <dt>Registration Deadline</dt>
+            <dd>{{ exchange.registration_close_date.strftime('%Y-%m-%d') }}</dd>
+        </dl>
+    </section>
+
+    <section>
+        <h2>Register</h2>
+        <form method="POST">
+            {{ form.hidden_tag() }}
+
+            <label for="name">
+                {{ form.name.label }}
+                {{ form.name(placeholder="Your name") }}
+                {% if form.name.errors %}
+                    <small>{{ form.name.errors[0] }}</small>
+                {% endif %}
+            </label>
+
+            <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>
+
+            <label for="gift_ideas">
+                {{ form.gift_ideas.label }}
+                {{ form.gift_ideas(placeholder="Things you like, hobbies, interests...", rows="4") }}
+                {% if form.gift_ideas.description %}
+                    <small>{{ form.gift_ideas.description }}</small>
+                {% endif %}
+                {% if form.gift_ideas.errors %}
+                    <small>{{ form.gift_ideas.errors[0] }}</small>
+                {% endif %}
+            </label>
+
+            <label for="reminder_enabled">
+                {{ form.reminder_enabled() }}
+                {{ form.reminder_enabled.label }}
+                {% if form.reminder_enabled.description %}
+                    <small>{{ form.reminder_enabled.description }}</small>
+                {% endif %}
+            </label>
+
+            <button type="submit">Register</button>
+        </form>
+    </section>
+</article>
+{% endblock %}