[Goodnight77]

Closes #193

Adds QdrantMemoryService, a BaseMemoryService backed by Qdrant vector search, scoped by app_name and user_id. qdrant-client is an optional qdrant extra.

Testing plan

Unit tests: tests/unittests/memory/test_qdrant_memory_service.py (mocked client plus one test on qdrant-client's in-process mode).

pytest tests/unittests
67 passed, 14 skipped

Manual E2E: a Runner turn with the service, then add, search and delete against Qdrant v1.15.4 in Docker:

[run] assistant: Noted, your favourite colour is teal.
[store] points in collection after adding session twice: 2
[search alice] author=user text='My favourite colour is teal.'
[search alice] author=assistant text='Noted, your favourite colour is teal.'
[search bob] results: 0
[delete as bob] alice still has 2 memories
[delete as alice] remaining: 0
E2E OK