fix(zigbee): parse real Z2M networkmap shape (data.value.nodes/links)

The previous parser read `data.routes` which is just an echo of the
`routes` request flag (a boolean). On real brokers this caused
`TypeError: 'bool' object is not iterable` and 500s during /import.

- Rewrite parse_networkmap to read data.value.nodes + data.value.links
  with fallback to data.{nodes,links} for legacy variants
- Defensive: drop links to unknown nodes, propagate lqi from link to
  target node, extract model/vendor from definition block
- Bump networkmap timeout 10s -> 180s (large meshes are slow)
- Tests: rewrite fixture builders + sample payload to real Z2M shape;
  add cases for legacy shape, routes:false echo (regression), malformed
  list, link to unknown node, lqi propagation, definition extraction
- Update docs to mention 60s+ wait window

53 backend tests pass, mypy + ruff clean.

backend/app/api/routes/zigbee.py

@@ -27,8 +27,8 @@ async def import_zigbee_network(
     """Fetch the Zigbee2MQTT network map and return nodes + edges ready for canvas drop.
 
     Connects to the specified MQTT broker, publishes a networkmap request to
-    ``<base_topic>/bridge/request/networkmap``, and waits up to 10 s for the
-    response.  The devices are returned as typed homelable nodes with a
+    ``<base_topic>/bridge/request/networkmap``, and waits up to 60 s for the
+    response (large meshes can take 30 s+).  The devices are returned as typed homelable nodes with a
     coordinator → router → end-device hierarchy.
     """
     try: