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/renderers/docx.py

@@ -160,12 +160,14 @@ def render_docx(doc):
                 cell = t.cell(0, i).paragraphs[0]
                 cell.alignment = WD_ALIGN_PARAGRAPH.CENTER
                 if png:
-                    cell.add_run().add_picture(BytesIO(png), width=Cm(7))  # Höhe skaliert mit der Rackgröße
+                    # Topologie: volle Breite; Racks: 7 cm (Höhe skaliert mit der Rackgröße)
+                    cell.add_run().add_picture(BytesIO(png), width=Cm(16.5 if blk.level == 1 else 7))
                 else:
                     cell.add_run("(Rack-Grafik nicht verfügbar)")
                 cap = t.cell(1, i).paragraphs[0]
                 cap.alignment = WD_ALIGN_PARAGRAPH.CENTER
-                cap.add_run(img["caption"]).italic = True
+                if img["caption"]:
+                    cap.add_run(img["caption"]).italic = True
             d.add_paragraph()
 
     # Fußzeile nur setzen, wenn die Vorlage keine eigene mitbringt