Delete a fact from a POT

Delete a fact from a POT.

Also deletes all edges connected to this fact (cascade delete) and its embedding in the vector store. The embedding is deleted FIRST (with retries): if the vector store is unavailable the request fails with 502 and the fact stays intact, so a client retry deletes both. The reverse order could leave a deleted fact still retrievable via semantic search.

• pot_id: UUID of the POT
• fact_id: UUID of the fact to delete