fix: use absolute paths for database to ensure persistence

Critical bug fix: Path(__file__).parent.parent returns a relative path,
causing the database to be created in different locations depending on
the working directory. This caused data loss on container restarts.

Changes:
- Add .resolve() to BASE_DIR in src/config.py and migrations/env.py
- Fix admin dashboard showing 0 for participant count (was hardcoded)
- Fix exchange detail page to show actual participant list
- Add startup diagnostics to entrypoint.sh for troubleshooting

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

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

src/templates/admin/exchange_detail.html

@@ -37,8 +37,30 @@
         </div>
 
         <div class="detail-section">
-            <h2>Participants</h2>
+            <h2>Participants ({{ exchange.participants|selectattr('withdrawn_at', 'none')|list|length }} / {{ exchange.max_participants }})</h2>
+            {% set active_participants = exchange.participants|selectattr('withdrawn_at', 'none')|list %}
+            {% if active_participants %}
+            <table>
+                <thead>
+                    <tr>
+                        <th>Name</th>
+                        <th>Email</th>
+                        <th>Registered</th>
+                    </tr>
+                </thead>
+                <tbody>
+                    {% for participant in active_participants|sort(attribute='name') %}
+                    <tr>
+                        <td>{{ participant.name }}</td>
+                        <td>{{ participant.email }}</td>
+                        <td>{{ participant.created_at.strftime('%Y-%m-%d %H:%M') }}</td>
+                    </tr>
+                    {% endfor %}
+                </tbody>
+            </table>
+            {% else %}
             <p>No participants yet.</p>
+            {% endif %}
         </div>
 
         <div class="actions">