Find memory items by exact metadata match. 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).