fix: allow DocMetadata to accept extra keyword arguments from stored payloads

[octo-patch]

Fixes #1374

Problem

When users add custom fields (e.g. filename) to document metadata before storing documents in Qdrant, retrieving those documents fails with:

TypeError: DocMetadata.__init__() got an unexpected keyword argument 'filename'

This happens because DocMetadata(**point.payload) is called in _qdrant_store.py during retrieval, but DocMetadata only accepts its four defined fields and rejects any extra keyword arguments.

Solution

Change DocMetadata to use @dataclass(init=False) and define a custom __init__ that accepts **kwargs. Any extra fields are stored as instance attributes via setattr, preserving user-defined metadata and making it accessible after retrieval.

Testing

Manually verified: after setting a custom filename attribute on DocMetadata and storing a document, calling knowledge.retrieve() no longer raises a TypeError and the filename attribute is accessible on the returned document's metadata.

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