added price drops to alerts/fixed race condition where late claimed docs got their tags wiped and sent to the api without tags

tests/test_new_listing_stamp.py

@@ -99,6 +99,32 @@ def test_existing_doc_with_new_price_stamps_false():
     assert alert_doc(collection)["newListing"] is False
 
 
+def test_drop_alert_carries_price_before_drop():
+    """the doc's priceBeforeDrop is cleared by the next no-drop cycle, so the
+    alert keeps its own copy from emit time — a drop claimed late still knows
+    what it dropped from"""
+
+    existing = {"_id": "p1", "currentPrice": 100.0, "pastPrices": []}
+    collection = FakeCollection([existing])
+    run(collection, [_product(current=95.0)])
+
+    assert alert_doc(collection)["priceBeforeDrop"] == 100.0
+
+
+def test_non_drop_alert_carries_none():
+    """a never-seen higher price alerts without a drop; discoveries have no
+    prior price at all"""
+
+    existing = {"_id": "p1", "currentPrice": 100.0, "pastPrices": []}
+    collection = FakeCollection([existing])
+    run(collection, [_product(current=120.0)])
+    assert alert_doc(collection)["priceBeforeDrop"] is None
+
+    discovery = FakeCollection()
+    run(discovery, [_product()])
+    assert alert_doc(discovery)["priceBeforeDrop"] is None
+
+
 def test_existing_doc_with_known_price_emits_no_alert():
     existing = {
         "_id": "p1",
@@ -112,8 +138,9 @@ def test_existing_doc_with_known_price_emits_no_alert():
     assert collection.alerts is None
 
 
-def test_build_alert_defaults_missing_flag_to_false():
+def test_build_alert_defaults_missing_fields():
     """Direct callers passing an unstamped document must not KeyError."""
 
     alert = build_alert("bestbuy", {"_id": "p1"}, NOW)
     assert alert["newListing"] is False
+    assert alert["priceBeforeDrop"] is None