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/symbol_views.py

@@ -0,0 +1,96 @@
+"""Verwaltung der Topologie-Symbole je Gerätetyp / Geräterolle."""
+from django.contrib import messages
+from django.contrib.auth.mixins import LoginRequiredMixin, PermissionRequiredMixin
+from django.http import Http404, HttpResponse
+from django.shortcuts import get_object_or_404, redirect, render
+from django.urls import reverse
+from django.views import View
+
+from .forms import TopologySymbolForm
+from .models import TopologySymbol
+from .symbols import HIDDEN, KINDS, builtin_icon
+
+APP = "netbox_customer_export"
+
+
+def _builtin_kinds():
+    return [(k, label, reverse(f"plugins:{APP}:symbol_builtin", kwargs={"kind": k})) for k, (label, _) in KINDS.items()]
+
+
+class BuiltinIconView(LoginRequiredMixin, View):
+    """PNG eines eingebauten Symbols (für Galerie und Vorschau)."""
+
+    def get(self, request, kind):
+        if kind not in KINDS:
+            raise Http404
+        response = HttpResponse(builtin_icon(kind), content_type="image/png")
+        response["Cache-Control"] = "max-age=86400"
+        return response
+
+
+class SymbolListView(PermissionRequiredMixin, View):
+    permission_required = f"{APP}.view_topologysymbol"
+
+    def get(self, request):
+        symbols = TopologySymbol.objects.select_related("device_type__manufacturer", "role")
+        builtin = {k: url for k, _, url in _builtin_kinds()}
+        rows = []
+        for s in symbols:
+            if s.icon:
+                preview = s.icon.url
+            elif s.kind and s.kind != HIDDEN:
+                preview = builtin[s.kind]
+            else:
+                preview = None
+            rows.append((s, preview))
+        return render(
+            request,
+            f"{APP}/symbol_list.html",
+            {"rows": rows, "builtin": _builtin_kinds(), "types": [r for r in rows if r[0].device_type_id], "roles": [r for r in rows if r[0].role_id]},
+        )
+
+
+class SymbolEditView(PermissionRequiredMixin, View):
+    template_name = f"{APP}/symbol_edit.html"
+
+    def get_permission_required(self):
+        return [f"{APP}.{'change' if self.kwargs.get('pk') else 'add'}_topologysymbol"]
+
+    def _obj(self, pk):
+        return get_object_or_404(TopologySymbol, pk=pk) if pk else TopologySymbol()
+
+    def _render(self, request, obj, form):
+        return render(request, self.template_name, {"object": obj, "form": form, "builtin": _builtin_kinds()})
+
+    def get(self, request, pk=None):
+        obj = self._obj(pk)
+        initial = {}
+        for key in ("device_type", "role"):  # Vorbelegung z.B. aus dem Link auf der Gerätetyp-Seite
+            if request.GET.get(key):
+                initial[key] = request.GET[key]
+        return self._render(request, obj, TopologySymbolForm(instance=obj, initial=initial))
+
+    def post(self, request, pk=None):
+        obj = self._obj(pk)
+        form = TopologySymbolForm(request.POST, request.FILES, instance=obj)
+        if form.is_valid():
+            obj = form.save()
+            messages.success(request, f"Topologie-Symbol für „{obj}“ gespeichert.")
+            return redirect(f"plugins:{APP}:symbol_list")
+        return self._render(request, obj, form)
+
+
+class SymbolDeleteView(PermissionRequiredMixin, View):
+    permission_required = f"{APP}.delete_topologysymbol"
+
+    def get(self, request, pk):
+        return render(request, f"{APP}/symbol_delete.html", {"object": get_object_or_404(TopologySymbol, pk=pk)})
+
+    def post(self, request, pk):
+        obj = get_object_or_404(TopologySymbol, pk=pk)
+        name = str(obj)
+        if obj.icon:
+            obj.icon.delete(save=False)
+        obj.delete()
+        messages.success(request, f"Topologie-Symbol für „{name}“ gelöscht – es gilt wieder die automatische Erkennung.")
+        return redirect(f"plugins:{APP}:symbol_list")