feat: add product sold event handling

Add sell_product() service method that creates ProductSold events
with calculated unit_price_cents (floor division of total/qty).
Update ProductsProjection to handle PRODUCT_SOLD events.

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

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

tests/test_service_products_sold.py

@@ -0,0 +1,200 @@
+# ABOUTME: Tests for ProductService sell_product operations.
+# ABOUTME: Tests sell_product with event creation and unit_price_cents calculation.
+
+import time
+
+import pytest
+
+from animaltrack.events.payloads import ProductSoldPayload
+from animaltrack.events.store import EventStore
+from animaltrack.events.types import PRODUCT_SOLD
+from animaltrack.projections import ProjectionRegistry
+from animaltrack.projections.products import ProductsProjection
+from animaltrack.services.products import ProductService, ValidationError
+
+
+@pytest.fixture
+def event_store(seeded_db):
+    """Create an EventStore for testing."""
+    return EventStore(seeded_db)
+
+
+@pytest.fixture
+def projection_registry(seeded_db):
+    """Create a ProjectionRegistry with ProductsProjection."""
+    registry = ProjectionRegistry()
+    registry.register(ProductsProjection(seeded_db))
+    return registry
+
+
+@pytest.fixture
+def product_service(seeded_db, event_store, projection_registry):
+    """Create a ProductService for testing."""
+    return ProductService(seeded_db, event_store, projection_registry)
+
+
+def make_sell_payload(
+    product_code: str = "egg.duck",
+    quantity: int = 30,
+    total_price_cents: int = 1500,
+    buyer: str | None = None,
+    notes: str | None = None,
+) -> ProductSoldPayload:
+    """Create a product sold payload for testing."""
+    return ProductSoldPayload(
+        product_code=product_code,
+        quantity=quantity,
+        total_price_cents=total_price_cents,
+        buyer=buyer,
+        notes=notes,
+    )
+
+
+# =============================================================================
+# sell_product Tests
+# =============================================================================
+
+
+class TestProductServiceSell:
+    """Tests for sell_product()."""
+
+    def test_creates_product_sold_event(self, seeded_db, product_service):
+        """sell_product creates a ProductSold event."""
+        payload = make_sell_payload()
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        assert event.type == PRODUCT_SOLD
+        assert event.actor == "test_user"
+        assert event.ts_utc == ts_utc
+
+    def test_event_has_product_code_in_entity_refs(self, seeded_db, product_service):
+        """Event entity_refs contains product_code."""
+        payload = make_sell_payload(product_code="egg.duck")
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        assert event.entity_refs["product_code"] == "egg.duck"
+
+    def test_event_has_quantity_in_entity_refs(self, seeded_db, product_service):
+        """Event entity_refs contains quantity."""
+        payload = make_sell_payload(quantity=24)
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        assert event.entity_refs["quantity"] == 24
+
+    def test_event_has_total_price_cents_in_entity_refs(self, seeded_db, product_service):
+        """Event entity_refs contains total_price_cents."""
+        payload = make_sell_payload(total_price_cents=2400)
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        assert event.entity_refs["total_price_cents"] == 2400
+
+    def test_event_has_unit_price_cents_in_entity_refs(self, seeded_db, product_service):
+        """Event entity_refs contains calculated unit_price_cents."""
+        payload = make_sell_payload(quantity=30, total_price_cents=1500)
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        # 1500 / 30 = 50 cents per unit
+        assert event.entity_refs["unit_price_cents"] == 50
+
+    def test_unit_price_cents_floor_division(self, seeded_db, product_service):
+        """unit_price_cents uses floor division (exact case)."""
+        # 1500 cents / 30 = 50.0 cents exactly
+        payload = make_sell_payload(quantity=30, total_price_cents=1500)
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        assert event.entity_refs["unit_price_cents"] == 50
+
+    def test_unit_price_cents_rounds_down(self, seeded_db, product_service):
+        """unit_price_cents rounds down for fractional cents."""
+        # 1000 cents / 3 = 333.33... -> 333
+        payload = make_sell_payload(quantity=3, total_price_cents=1000)
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        assert event.entity_refs["unit_price_cents"] == 333
+
+    def test_unit_price_cents_another_fractional_case(self, seeded_db, product_service):
+        """unit_price_cents correctly floors another fractional case."""
+        # 1001 cents / 3 = 333.66... -> 333
+        payload = make_sell_payload(quantity=3, total_price_cents=1001)
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        assert event.entity_refs["unit_price_cents"] == 333
+
+    def test_event_stored_in_events_table(self, seeded_db, product_service):
+        """Event is stored in the events table."""
+        payload = make_sell_payload()
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        row = seeded_db.execute(
+            "SELECT id, type FROM events WHERE id = ?",
+            (event.id,),
+        ).fetchone()
+
+        assert row is not None
+        assert row[0] == event.id
+        assert row[1] == PRODUCT_SOLD
+
+    def test_buyer_stored_in_payload(self, seeded_db, product_service):
+        """Optional buyer is stored in event payload."""
+        payload = make_sell_payload(buyer="Local Market")
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        assert event.payload["buyer"] == "Local Market"
+
+    def test_notes_stored_in_payload(self, seeded_db, product_service):
+        """Optional notes are stored in event payload."""
+        payload = make_sell_payload(notes="Cash sale")
+        ts_utc = int(time.time() * 1000)
+
+        event = product_service.sell_product(payload, ts_utc, "test_user")
+
+        assert event.payload["notes"] == "Cash sale"
+
+
+class TestProductServiceSellValidation:
+    """Tests for sell_product() validation."""
+
+    def test_rejects_nonexistent_product(self, seeded_db, product_service):
+        """Raises ValidationError for non-existent product_code."""
+        payload = make_sell_payload(product_code="nonexistent.product")
+
+        with pytest.raises(ValidationError, match="not found"):
+            product_service.sell_product(payload, int(time.time() * 1000), "test_user")
+
+    def test_rejects_inactive_product(self, seeded_db, product_service):
+        """Raises ValidationError for inactive product."""
+        seeded_db.execute("UPDATE products SET active = 0 WHERE code = 'egg.duck'")
+
+        payload = make_sell_payload(product_code="egg.duck")
+
+        with pytest.raises(ValidationError, match="inactive"):
+            product_service.sell_product(payload, int(time.time() * 1000), "test_user")
+
+    def test_rejects_non_sellable_product(self, seeded_db, product_service):
+        """Raises ValidationError for non-sellable product."""
+        seeded_db.execute("UPDATE products SET sellable = 0 WHERE code = 'egg.duck'")
+
+        payload = make_sell_payload(product_code="egg.duck")
+
+        with pytest.raises(ValidationError, match="not sellable"):
+            product_service.sell_product(payload, int(time.time() * 1000), "test_user")