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_e2e/test_price_history.py

@@ -0,0 +1,102 @@
+"""E2E: Price history queries returning correct data."""
+
+import pytest
+
+
+@pytest.mark.asyncio
+class TestPriceTrends:
+    """Verify price trend aggregation against seeded history."""
+
+    async def test_trends_returns_all_products(self, client, seed_data):
+        resp = await client.get("/prices/trends", headers=seed_data["headers"])
+        assert resp.status_code == 200
+        data = resp.json()
+        product_names = [t["product_name"] for t in data]
+        assert "Cheerios 18oz" in product_names
+        assert "Whole Milk 1gal" in product_names
+
+    async def test_trends_filter_by_category(self, client, seed_data):
+        resp = await client.get(
+            "/prices/trends", params={"category": "dairy"}, headers=seed_data["headers"]
+        )
+        assert resp.status_code == 200
+        data = resp.json()
+        assert len(data) >= 1
+        # Only dairy products should appear
+        for trend in data:
+            assert trend["product_name"] == "Whole Milk 1gal"
+
+    async def test_trends_contain_data_points(self, client, seed_data):
+        resp = await client.get("/prices/trends", headers=seed_data["headers"])
+        data = resp.json()
+        cheerios_trend = next(t for t in data if t["product_name"] == "Cheerios 18oz")
+        assert len(cheerios_trend["data_points"]) >= 3
+
+
+@pytest.mark.asyncio
+class TestPriceIncreases:
+    """Detect price increases from seeded price history."""
+
+    async def test_increases_detected(self, client, seed_data):
+        resp = await client.get("/prices/increases", headers=seed_data["headers"])
+        assert resp.status_code == 200
+        data = resp.json()
+        # Cheerios at Meijer went from 3.99 → 4.29 → 4.79
+        cheerios_increases = [inc for inc in data if inc["product_name"] == "Cheerios 18oz"]
+        assert len(cheerios_increases) >= 1
+        # Verify the increase data makes sense
+        for inc in cheerios_increases:
+            assert inc["new_price"] > inc["old_price"]
+            assert inc["increase_pct"] > 0
+            assert inc["store_name"] == "Meijer"
+
+    async def test_stable_prices_not_flagged(self, client, seed_data):
+        """Kroger Cheerios price is stable at $4.49 — should not appear as increase."""
+        resp = await client.get("/prices/increases", headers=seed_data["headers"])
+        data = resp.json()
+        kroger_increases = [
+            inc
+            for inc in data
+            if inc["product_name"] == "Cheerios 18oz" and inc["store_name"] == "Kroger"
+        ]
+        assert len(kroger_increases) == 0
+
+
+@pytest.mark.asyncio
+class TestPriceComparison:
+    """Compare prices across stores for specific products."""
+
+    async def test_compare_cheerios_across_stores(self, client, seed_data):
+        cheerios_id = str(seed_data["products"]["cheerios"].id)
+        resp = await client.get(
+            "/prices/comparison",
+            params={"product_ids": cheerios_id},
+            headers=seed_data["headers"],
+        )
+        assert resp.status_code == 200
+        data = resp.json()
+        assert len(data) >= 1
+        cheerios_cmp = data[0]
+        assert cheerios_cmp["product_name"] == "Cheerios 18oz"
+        store_names = [p["store_name"] for p in cheerios_cmp["prices"]]
+        assert "Meijer" in store_names
+        assert "Kroger" in store_names
+
+    async def test_compare_requires_product_ids(self, client, seed_data):
+        """product_ids is required — omitting it must return 422."""
+        resp = await client.get("/prices/comparison", headers=seed_data["headers"])
+        assert resp.status_code == 422
+
+    async def test_compare_multiple_products(self, client, seed_data):
+        cheerios_id = str(seed_data["products"]["cheerios"].id)
+        milk_id = str(seed_data["products"]["milk"].id)
+        resp = await client.get(
+            "/prices/comparison",
+            params=[("product_ids", cheerios_id), ("product_ids", milk_id)],
+            headers=seed_data["headers"],
+        )
+        assert resp.status_code == 200
+        data = resp.json()
+        names = [c["product_name"] for c in data]
+        assert "Cheerios 18oz" in names
+        assert "Whole Milk 1gal" in names