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_middleware/test_error_handler.py

@@ -0,0 +1,54 @@
+"""Tests for structured error responses and error monitoring."""
+
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_404_returns_structured_error(client):
+    """Non-existent route should return structured error."""
+    resp = await client.get("/nonexistent")
+    assert resp.status_code == 404
+    body = resp.json()
+    assert "detail" in body
+    assert "code" in body
+    assert body["code"] == "NOT_FOUND"
+
+
+@pytest.mark.asyncio
+async def test_validation_error_returns_422_with_field_errors(client):
+    """Invalid request body should return structured validation errors."""
+    resp = await client.post(
+        "/auth/register",
+        json={"email": "not-an-email", "password": "short", "display_name": ""},
+    )
+    assert resp.status_code == 422
+    body = resp.json()
+    assert body["code"] == "VALIDATION_ERROR"
+    assert "errors" in body
+    assert isinstance(body["errors"], list)
+    assert len(body["errors"]) > 0
+    # Each error should have field, message, type
+    for err in body["errors"]:
+        assert "field" in err
+        assert "message" in err
+        assert "type" in err
+
+
+@pytest.mark.asyncio
+async def test_error_stats_requires_service_key(client):
+    """Error stats endpoint should require X-Service-Key."""
+    resp = await client.get("/internal/error-stats")
+    assert resp.status_code == 422  # Missing required header
+
+
+@pytest.mark.asyncio
+async def test_error_stats_with_valid_key(client):
+    """Error stats endpoint returns monitoring data with valid key."""
+    resp = await client.get(
+        "/internal/error-stats",
+        headers={"X-Service-Key": "change-me-in-production"},
+    )
+    assert resp.status_code == 200
+    body = resp.json()
+    assert "error_counts" in body
+    assert "recent_5xx_count" in body