[jihtsan]

A source refresh could finish fetching after DELETE returned and put the removed file and vectors back. Re-registering the same collection could also schedule duplicate refreshes.

Make collection registration idempotent and check that a refresh still belongs to the same source before storing. A collection lock orders refresh writes and removals without blocking source operations in other collections.

Closes #32.

Validation:

• Three deterministic regressions cover a late fetch after deletion, duplicate scheduling, and deletion in one collection while another collection's write is blocked. Ran them 10 times with the race detector. The late-fetch test uses a real local Chromem store and a stub embeddings endpoint.
• go test -race -count=1 ./rag ./rag/engine -run 'TestSource|TestMemoryChromem' and go vet ./rag ./rag/engine . passed.
• go test -count=1 ./... reports the same failures on the clean base and this change: unavailable LocalAI/PostgreSQL services and pre-existing function-signature errors in test/raw. No real model was used, and this change does not coordinate source deletion across server replicas.