Add run deletion with filesystem cleanup

- New POST /run/<id>/delete route removes run from DB and deletes run directory
- Only allows deletion of non-active runs (blocks running/queued)
- Delete Run button shown in run_details.html header for non-active runs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/run_details.html

@@ -16,7 +16,15 @@
         <button class="btn btn-primary me-2" data-bs-toggle="modal" data-bs-target="#modifyRunModal" data-bs-toggle="tooltip" title="Create a new run based on this one, but with different instructions (e.g. 'Make it darker').">
             <i class="fas fa-pen-fancy me-2"></i>Modify & Re-run
         </button>
-        <a href="{{ url_for('project.view_project', id=run.project_id) }}" class="btn btn-outline-secondary">Back to Project</a>
+        {% if run.status not in ['running', 'queued'] %}
+        <form action="{{ url_for('run.delete_run', id=run.id) }}" method="POST" class="d-inline ms-2"
+              onsubmit="return confirm('Delete Run #{{ run.id }} and all its files? This cannot be undone.');">
+            <button type="submit" class="btn btn-outline-danger">
+                <i class="fas fa-trash me-2"></i>Delete Run
+            </button>
+        </form>
+        {% endif %}
+        <a href="{{ url_for('project.view_project', id=run.project_id) }}" class="btn btn-outline-secondary ms-2">Back to Project</a>
     </div>
 </div>