feat: merge cartsnitch/api into api/ subdirectory

Consolidate API gateway service into monorepo.
Squashed from https://github.com/cartsnitch/api main (89bacb1).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

tests/test_routes/test_coupons.py

@@ -0,0 +1,58 @@
+"""Integration tests for coupon endpoints."""
+
+from datetime import date
+from decimal import Decimal
+
+import pytest
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
+
+from cartsnitch_api.models import Coupon, Store
+
+
+@pytest.fixture
+async def coupon_data(db_engine, auth_headers):
+    """Seed stores and coupons."""
+    factory = async_sessionmaker(db_engine, class_=AsyncSession, expire_on_commit=False)
+    async with factory() as session:
+        store = Store(name="Target", slug="target")
+        session.add(store)
+        await session.commit()
+        await session.refresh(store)
+
+        coupon = Coupon(
+            store_id=store.id,
+            title="$2 off laundry",
+            description="$2 off any laundry detergent",
+            discount_value=Decimal("2.00"),
+            discount_type="fixed",
+            valid_from=date(2026, 1, 1),
+            valid_to=date(2026, 12, 31),
+        )
+        session.add(coupon)
+        await session.commit()
+
+        return {"store": store, "coupon": coupon, "headers": auth_headers}
+
+
+@pytest.mark.asyncio
+async def test_list_coupons(client, coupon_data):
+    resp = await client.get("/coupons", headers=coupon_data["headers"])
+    assert resp.status_code == 200
+    data = resp.json()
+    assert len(data) >= 1
+
+
+@pytest.mark.asyncio
+async def test_list_coupons_by_store(client, coupon_data):
+    store_id = str(coupon_data["store"].id)
+    resp = await client.get(f"/coupons?store_id={store_id}", headers=coupon_data["headers"])
+    assert resp.status_code == 200
+    assert len(resp.json()) >= 1
+
+
+@pytest.mark.asyncio
+async def test_relevant_coupons_empty(client, auth_headers):
+    """No purchases means no relevant coupons."""
+    resp = await client.get("/coupons/relevant", headers=auth_headers)
+    assert resp.status_code == 200
+    assert resp.json() == []