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/blocks.py

@@ -52,7 +52,7 @@ def KV(pairs, skip_empty=True):
     return Block("kv", rows=rows)
 
 
-def IMAGES(images):
+def IMAGES(images, wide=False):
     """images: Liste von (svg_string oder None, caption, png_bytes_oder_None)."""
     out = []
     for svg, caption, png in images:
@@ -63,7 +63,7 @@ def IMAGES(images):
         else:
             continue
         out.append({"svg": svg, "png": png, "caption": caption, "data_uri": data_uri})
-    return Block("images", images=out)
+    return Block("images", images=out, level=1 if wide else 0)  # level=1: volle Seitenbreite
 
 
 def PAGEBREAK():