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.

docs/zigbee-import.md

@@ -55,7 +55,7 @@ Click **Fetch Devices**. Homelable will:
 1. Connect to the broker
 2. Subscribe to the response topic
 3. Publish `{"type": "raw", "routes": false}` to the request topic
-4. Wait up to 10 seconds for the network map response
+4. Wait up to 60 seconds for the network map response (large meshes can take 30 s+)
 5. Parse and group devices by type
 
 ### 5. Select and add to canvas