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/migrations/0002_topologysymbol.py

@@ -0,0 +1,28 @@
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("dcim", "__first__"),
+        ("netbox_customer_export", "0001_brandingprofile"),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name="TopologySymbol",
+            fields=[
+                ("id", models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name="ID")),
+                ("kind", models.CharField(blank=True, max_length=20, verbose_name="Symbol")),
+                ("icon", models.ImageField(blank=True, upload_to="customer-export/symbols/", verbose_name="Eigenes Icon")),
+                ("last_updated", models.DateTimeField(auto_now=True)),
+                ("device_type", models.OneToOneField(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name="+", to="dcim.devicetype", verbose_name="Gerätetyp")),
+                ("role", models.OneToOneField(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name="+", to="dcim.devicerole", verbose_name="Geräterolle")),
+            ],
+            options={
+                "verbose_name": "Topologie-Symbol",
+                "verbose_name_plural": "Topologie-Symbole",
+                "ordering": ("device_type__model", "role__name"),
+            },
+        ),
+    ]