Add a memory item with optional full ingestion pipeline. Requires authentication and operates on user-scoped data. Args: content: The memory content memory_type: Type of memory (semantic, episodic, procedural) metadata: Additional metadata use_pipeline: If True, uses full ingestion pipeline with entity extraction (default: True) conversation_context: Optional conversation context for conversation-aware entity extraction embed: SVC-EMBED-CONTROL-1 tri-state embedding override. True forces an embedding, False suppresses one, None (default) leaves existing behaviour untouched. Only honoured when `use_pipeline` is False — see the 400 below. Note: Provenance like `retrieved_context_ids` travels in `metadata` — the storage layer flattens metadata onto the node and lifts known fields back onto the MemoryItem on read, so no dedicated body field is needed (GRAPH-API-1i, wire field added then removed 2026-08-02; metadata takes precedence at storage regardless, node_types.py).