feat: implement exchange creation

Add Exchange model, API endpoint, and form validation
for creating new gift exchanges.

- Create ExchangeForm with timezone validation
- Add admin routes for creating and viewing exchanges
- Generate unique 12-char slug for each exchange
- Validate registration/exchange dates
- Display exchanges in dashboard
- All tests passing with 92% coverage

Story: 2.1

src/templates/admin/exchange_detail.html

@@ -0,0 +1,58 @@
+{% extends "layouts/base.html" %}
+
+{% block title %}{{ exchange.name }}{% endblock %}
+
+{% block content %}
+<div class="container">
+    <h1>{{ exchange.name }}</h1>
+
+    <div class="exchange-details">
+        <div class="detail-section">
+            <h2>Details</h2>
+            <dl>
+                <dt>State</dt>
+                <dd><span class="badge badge-{{ exchange.state }}">{{ exchange.state }}</span></dd>
+
+                <dt>Description</dt>
+                <dd>{{ exchange.description or "No description" }}</dd>
+
+                <dt>Budget</dt>
+                <dd>{{ exchange.budget }}</dd>
+
+                <dt>Max Participants</dt>
+                <dd>{{ exchange.max_participants }}</dd>
+
+                <dt>Registration Close Date</dt>
+                <dd>{{ exchange.registration_close_date.strftime('%Y-%m-%d %H:%M') }} {{ exchange.timezone }}</dd>
+
+                <dt>Exchange Date</dt>
+                <dd>{{ exchange.exchange_date.strftime('%Y-%m-%d %H:%M') }} {{ exchange.timezone }}</dd>
+
+                <dt>Registration Link</dt>
+                <dd>
+                    <code id="registration-link">{{ url_for('participant.register', slug=exchange.slug, _external=True) }}</code>
+                    <button type="button" onclick="copyToClipboard()">Copy</button>
+                </dd>
+            </dl>
+        </div>
+
+        <div class="detail-section">
+            <h2>Participants</h2>
+            <p>No participants yet.</p>
+        </div>
+
+        <div class="actions">
+            <a href="{{ url_for('admin.dashboard') }}" class="btn btn-secondary">Back to Dashboard</a>
+        </div>
+    </div>
+</div>
+
+<script>
+function copyToClipboard() {
+    const link = document.getElementById('registration-link').textContent;
+    navigator.clipboard.writeText(link).then(() => {
+        alert('Link copied to clipboard!');
+    });
+}
+</script>
+{% endblock %}