fix(memory): sweep legacy orphaned message vectors

Repair vector rows left behind by older databases at store startup. Keep the existing single-statement delete path from main and avoid reintroducing the redundant batched helper.

tests/test_documents.py

@@ -136,6 +136,27 @@ def test_conversation_recall_uses_vec_and_matches_brute_force():
     asyncio.run(run())
 
 
+def test_startup_sweeps_pre_existing_orphan_vectors():
+    """Databases written before delete_conversation cleaned up after itself are
+    repaired the next time the store opens them."""
+    import json
+    path = Path(tempfile.mkdtemp()) / "t.db"
+    s = PersistentMemoryStore(path)
+    s.create_conversation("c1")
+    mid = s.add_message("c1", "user", "lego star wars")
+    conn = s._connect()
+    conn.execute("INSERT INTO message_vectors (message_id, embedding) VALUES (?, ?)",
+                 (mid, json.dumps([1.0, 0.0])))
+    conn.execute("DELETE FROM messages WHERE id = ?", (mid,))  # the old leaky delete
+    conn.commit()
+    conn.close()
+
+    reopened = PersistentMemoryStore(path)
+    conn = reopened._connect()
+    assert conn.execute("SELECT COUNT(*) FROM message_vectors").fetchone()[0] == 0
+    conn.close()
+
+
 def test_projects_scope_documents_and_survive_delete():
     s = _store()