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/sections.py

@@ -406,3 +406,18 @@ def licenses(scope, opts):
     if not found_source:
         return _empty(opts, blocks, "Kein Lizenz-Plugin installiert (z.B. netbox-lifecycle) – siehe README.")
     return blocks if len(blocks) > 1 else _empty(opts, blocks)
+
+
+@section("topology", "Netzwerk-Topologie", "Netzwerk", default=False, hint="Diagramm mit Symbolen, wie Packet Tracer")
+def topology(scope, opts):
+    from .topology import build_graphs, render_png
+
+    blocks = [H(1, "Netzwerk-Topologie")]
+    graphs = build_graphs(scope, opts)
+    if not graphs:
+        return _empty(opts, blocks, "Keine verkabelten Geräte gefunden.")
+    ports = "ports" in (opts.get("topo_details") or [])
+    for g in graphs:
+        blocks.append(H(2, g.title))
+        blocks.append(IMAGES([(None, "", render_png(g, show_ports=ports))], wide=True))
+    return blocks