Allow recording zero eggs collected

Enable recording "checked coop, found 0 eggs" to distinguish from days
when the coop wasn't checked at all. Statistics remain eggs/calendar day.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_event_payloads.py

@@ -285,15 +285,27 @@ class TestProductPayloads:
         )
         assert payload.quantity == 12
 
-    def test_quantity_must_be_positive(self):
-        """quantity must be >= 1."""
+    def test_quantity_zero_is_valid(self):
+        """quantity=0 is valid (checked but found none)."""
+        from animaltrack.events.payloads import ProductCollectedPayload
+
+        payload = ProductCollectedPayload(
+            location_id="01ARZ3NDEKTSV4RRFFQ69G5FAV",
+            product_code="egg.duck",
+            quantity=0,
+            resolved_ids=["01ARZ3NDEKTSV4RRFFQ69G5FAV"],
+        )
+        assert payload.quantity == 0
+
+    def test_quantity_cannot_be_negative(self):
+        """quantity must be >= 0."""
         from animaltrack.events.payloads import ProductCollectedPayload
 
         with pytest.raises(ValidationError):
             ProductCollectedPayload(
                 location_id="01ARZ3NDEKTSV4RRFFQ69G5FAV",
                 product_code="egg.duck",
-                quantity=0,
+                quantity=-1,
                 resolved_ids=["01ARZ3NDEKTSV4RRFFQ69G5FAV"],
             )