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_rate_limit.py

@@ -0,0 +1,55 @@
+"""Tests for rate limiting middleware."""
+
+import pytest
+
+from cartsnitch_api.middleware.rate_limit import _SlidingWindowCounter
+
+
+class TestSlidingWindowCounter:
+    def test_allows_within_limit(self):
+        counter = _SlidingWindowCounter(max_requests=5, window_seconds=60)
+        for i in range(5):
+            allowed, remaining, retry = counter.is_allowed("test-key")
+            assert allowed is True
+            assert remaining == 4 - i
+
+    def test_blocks_over_limit(self):
+        counter = _SlidingWindowCounter(max_requests=3, window_seconds=60)
+        for _ in range(3):
+            counter.is_allowed("test-key")
+
+        allowed, remaining, retry = counter.is_allowed("test-key")
+        assert allowed is False
+        assert remaining == 0
+        assert retry > 0
+
+    def test_separate_keys(self):
+        counter = _SlidingWindowCounter(max_requests=2, window_seconds=60)
+        # Fill key-a
+        counter.is_allowed("key-a")
+        counter.is_allowed("key-a")
+        allowed_a, _, _ = counter.is_allowed("key-a")
+        assert allowed_a is False
+
+        # key-b should still be allowed
+        allowed_b, remaining, _ = counter.is_allowed("key-b")
+        assert allowed_b is True
+        assert remaining == 1
+
+
+@pytest.mark.asyncio
+async def test_rate_limit_returns_429(client):
+    """Public endpoint should return 429 after limit exceeded."""
+    # The default limit is 60/min — we won't hit it in normal tests,
+    # but we verify the middleware adds rate limit headers.
+    resp = await client.get("/public/inflation")
+    assert "x-ratelimit-limit" in resp.headers
+    assert "x-ratelimit-remaining" in resp.headers
+
+
+@pytest.mark.asyncio
+async def test_health_skips_rate_limit(client):
+    """Health endpoint should not have rate limit headers."""
+    resp = await client.get("/health")
+    assert resp.status_code == 200
+    assert "x-ratelimit-limit" not in resp.headers