fix: accept subchannel references as stored in the OSW backend

[simontaurus]

Problem

The OSW schema defines subchannels as an array of strings picked from the sibling data_channels by osw_id:

"subchannels": {
  "type": "array",
  "items": {
    "title": "Subchannel reference", "type": "string",
    "watch": {"_channels": "data_channels"},
    "enumSource": [{"source": "_channels", "value": "{{item.osw_id}}"}]
  }
}

Codegen honours that (_model.py: subchannels: list[str] | None), but both controller classes narrowed the field back to a list of OpcUaDataChannel objects. Loading a server from the backend therefore failed:

pydantic.v1.error_wrappers.ValidationError: 2 validation errors for OpcUaDataChannel
subchannels -> 0
  value is not a valid dict (type=type_error.dict)

_handle_datachange_notification calls read_channels(subchannels), which needs .node_id, so objects are still required at runtime.

Change

• Widen subchannels to Optional[List[Union[str, "OpcUaDataChannel"]]] in both the v1 and v2 controllers, so stored references load and inline objects keep working.
• Add OpcUaServerMixin.get_channel_by_osw_id(), which searches self and all subdevices and accepts both the full subobject IRI (Item:OSW<tool>#OSW<channel>) and the bare OSW<channel> suffix.
• Resolve string references in _handle_datachange_notification before reading them.

These are same-page subobject references, not cross-page ones, so range/__iris__ is not the right mechanism here: its lazy resolver loads entities by IRI, and there is no page behind Item:OSWdev#OSWchan.

Tests

6 new tests covering the IRI form, resolution across the subdevice hierarchy, the bare-suffix form, the unknown-id error, and a full serialize/reload/resolve round-trip. 47 pass in test_controller.py; flake8 and black clean.

Verified against live data: loading the vac-station hierarchy from the backend now yields 6 subdevices and 90 channels, with event_name_ebg1.subchannels resolving to ns=4;i=1222 and ns=4;i=1230.