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/dashboard.html

@@ -12,8 +12,56 @@
         </form>
     </header>
 
-    <p>Welcome to the Sneaky Klaus admin dashboard!</p>
+    <div class="dashboard-summary">
+        <div class="grid">
+            <div>
+                <h3>Draft</h3>
+                <p><strong>{{ draft_count }}</strong></p>
+            </div>
+            <div>
+                <h3>Active</h3>
+                <p><strong>{{ active_count }}</strong></p>
+            </div>
+            <div>
+                <h3>Completed</h3>
+                <p><strong>{{ completed_count }}</strong></p>
+            </div>
+        </div>
+    </div>
 
-    <p>This is a placeholder for the admin dashboard. More features coming soon.</p>
+    <div style="margin: 2rem 0;">
+        <a href="{{ url_for('admin.create_exchange') }}" role="button">Create New Exchange</a>
+    </div>
+
+    <h2>All Exchanges</h2>
+
+    {% if exchanges %}
+    <table>
+        <thead>
+            <tr>
+                <th>Name</th>
+                <th>State</th>
+                <th>Participants</th>
+                <th>Exchange Date</th>
+                <th>Actions</th>
+            </tr>
+        </thead>
+        <tbody>
+            {% for exchange in exchanges %}
+            <tr>
+                <td>{{ exchange.name }}</td>
+                <td><mark>{{ exchange.state }}</mark></td>
+                <td>0 / {{ exchange.max_participants }}</td>
+                <td>{{ exchange.exchange_date.strftime('%Y-%m-%d') }}</td>
+                <td>
+                    <a href="{{ url_for('admin.view_exchange', exchange_id=exchange.id) }}">View</a>
+                </td>
+            </tr>
+            {% endfor %}
+        </tbody>
+    </table>
+    {% else %}
+    <p>No exchanges yet. <a href="{{ url_for('admin.create_exchange') }}">Create your first exchange</a>!</p>
+    {% endif %}
 </article>
 {% endblock %}