Find memory items by exact metadata match. **Deprecated (GRAPH-API-1l):** use `GET /memory/list?metadata_key=<key>&metadata_value=<value>` instead. It applies the same filter but adds pagination and an exact `total`, which this route has never had. Behaviour here is unchanged and this endpoint still works — retirement is gated on a recorded production parity diff, because the two routes return different item shapes (this one carries `superseded` / `superseded_by` and synthesizes content for code rows). Returns a list of items whose `metadata.<metadata_key>` equals `metadata_value`. Empty result is HTTP 200 with `{items: [], count: 0}` — "no match" is not an error. Storage flattens metadata into top-level node properties (separator `__`), so `metadata_key="bot_topic"` matches the flat prop `n.bot_topic`. Nested keys can be passed as `nested.key` and are translated to `nested__key`. Previously this route hardcoded `top_k=1` and 404'd on empty; both have been corrected here. The core `search_by_metadata` was also rewritten (was returning `[]` for every call due to a missing method on SmartGraph).