feat: network topology diagram with Packet-Tracer-style symbols

- New section "Netzwerk-Topologie": one diagram per site built from
  NetBox cable paths (traced through patch panels), layered top-down by
  cabling (provider > firewall/router > core > access > endpoints),
  barycenter ordering, "2x" for parallel links, provider circuits as
  clouds and devices outside the scope dashed. Labels: management IP,
  model, ports.
- New export format "draw.io": the topology as an editable .drawio file
  (one page per site, symbols embedded).
- 14 built-in symbols drawn with Pillow; automatic mapping from role,
  model and manufacturer, passive gear (patch panels, PDUs, UPS) hidden.
- New TopologySymbol model (migration 0002) with list/edit/delete UI,
  symbol gallery, custom icon upload and a "Topologie-Symbol" button on
  device type and device role pages.

Bump version to 0.4.0 - requires "manage.py migrate".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

netbox_customer_export/template_content.py

@@ -18,4 +18,27 @@ class CustomerExportButton(PluginTemplateExtension):
         )
 
 
-template_extensions = [CustomerExportButton]
+class TopologySymbolButton(PluginTemplateExtension):
+    """Auf Gerätetyp- und Geräterollen-Seiten: Topologie-Symbol festlegen."""
+
+    models = ["dcim.devicetype", "dcim.devicerole"]
+
+    def buttons(self):
+        from .models import TopologySymbol
+
+        user = self.context["request"].user
+        if not user.has_perm("netbox_customer_export.add_topologysymbol"):
+            return ""
+        obj = self.context["object"]
+        field = "device_type" if obj._meta.model_name == "devicetype" else "role"
+        existing = TopologySymbol.objects.filter(**{field: obj}).first()
+        if existing:
+            url = existing.get_absolute_url()
+        else:
+            url = f"{reverse('plugins:netbox_customer_export:symbol_add')}?{field}={obj.pk}"
+        return format_html(
+            '<a href="{}" class="btn btn-outline-primary"><i class="mdi mdi-shape-outline"></i> Topologie-Symbol</a>', url
+        )
+
+
+template_extensions = [CustomerExportButton, TopologySymbolButton]