feat: Implement series splitting by DICOM tag with HTMX support

Co-authored-by: Copilot <copilot@github.com>

atlas/templates/atlas/partials/_series_split_by_tag.html

@@ -0,0 +1,17 @@
+{% load django_htmx %}
+<form method="post"
+      action="{% url 'atlas:series_split_by_tag' pk=series.pk %}"
+      hx-post="{% url 'atlas:series_split_by_tag' pk=series.pk %}"
+      hx-target="#split-by-tag-result"
+      hx-swap="innerHTML">
+    {% csrf_token %}
+    <div class="input-group input-group-sm">
+        <select name="dicom_tag" class="form-select form-select-sm">
+            {% for tag_value, tag_label in split_tags %}
+                <option value="{{ tag_value }}">{{ tag_label }}</option>
+            {% endfor %}
+        </select>
+        <button type="submit" class="btn btn-outline-warning btn-sm">Split</button>
+    </div>
+</form>
+<div id="split-by-tag-result" class="mt-2"></div>

atlas/templates/atlas/series_viewer.html

@@ -190,7 +190,11 @@
                         <a class="btn btn-outline-primary btn-sm" href="{% url 'atlas:series_order_dicom_instance' pk=series.pk %}">Order by instance number</a>
                         <a class="btn btn-outline-primary btn-sm" href="{% url 'atlas:series_order_dicom_SeriesInstanceUID' pk=series.pk %}">Order by SeriesInstanceUID</a>
                         <a class="btn btn-outline-primary btn-sm" href="{% url 'atlas:series_order_upload_filename' pk=series.pk %}">Order by uploaded filename</a>
-                        <a class="btn btn-outline-warning btn-sm" href="{% url 'api-1:series_split_by_tag' series.pk 'ImageType' %}">Split by ImageType tag</a>
+                        <div hx-get="{% url 'atlas:series_split_by_tag' pk=series.pk %}"
+                             hx-trigger="load"
+                             hx-swap="outerHTML">
+                            <span class="text-muted small">Loading split controls…</span>
+                        </div>
                     </div>
                 </div>