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_public_endpoints.py

@@ -0,0 +1,59 @@
+"""E2E: Public price transparency endpoints (no auth required)."""
+
+import uuid
+
+import pytest
+
+
+@pytest.mark.asyncio
+class TestPublicTrends:
+    """Public price trend endpoint — no auth, real data."""
+
+    async def test_public_trend_returns_data(self, client, seed_data):
+        cheerios_id = str(seed_data["products"]["cheerios"].id)
+        resp = await client.get(f"/public/trends/{cheerios_id}")
+        assert resp.status_code == 200
+        data = resp.json()
+        assert data["product_name"] == "Cheerios 18oz"
+        assert len(data["data_points"]) >= 3
+
+    async def test_public_trend_no_auth_needed(self, client, seed_data):
+        """Confirm no Authorization header is required."""
+        cheerios_id = str(seed_data["products"]["cheerios"].id)
+        resp = await client.get(f"/public/trends/{cheerios_id}")
+        assert resp.status_code == 200
+
+
+@pytest.mark.asyncio
+class TestPublicStoreComparison:
+    """Public store comparison endpoint."""
+
+    async def test_store_comparison(self, client, seed_data):
+        cheerios_id = str(seed_data["products"]["cheerios"].id)
+        resp = await client.get(
+            "/public/store-comparison",
+            params=[("product_ids", cheerios_id)],
+        )
+        assert resp.status_code == 200
+        data = resp.json()
+        assert "products" in data
+        assert len(data["products"]) >= 1
+
+    async def test_store_comparison_rejects_more_than_20_ids(self, client):
+        """max_length=20 guard: 21 product IDs must return 422."""
+        too_many = [("product_ids", str(uuid.uuid4())) for _ in range(21)]
+        resp = await client.get("/public/store-comparison", params=too_many)
+        assert resp.status_code == 422
+
+
+@pytest.mark.asyncio
+class TestPublicInflation:
+    """Public inflation index endpoint."""
+
+    async def test_inflation_returns_index(self, client, seed_data):
+        resp = await client.get("/public/inflation")
+        assert resp.status_code == 200
+        data = resp.json()
+        assert "cartsnitch_index" in data
+        assert "cpi_baseline" in data
+        assert "categories" in data