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>

src/cartsnitch_api/cache.py

@@ -0,0 +1,26 @@
+"""Redis/DragonflyDB caching helpers."""
+
+from cartsnitch_api.config import settings
+
+
+class CacheClient:
+    """Stub for Redis/DragonflyDB caching.
+
+    Will be used for expensive queries: price trends, product comparisons.
+    Cache invalidation via Redis pub/sub events from other services.
+    """
+
+    def __init__(self) -> None:
+        self.url = settings.redis_url
+
+    async def get(self, key: str) -> str | None:
+        # TODO: implement with redis-py async
+        return None
+
+    async def set(self, key: str, value: str, ttl_seconds: int = 300) -> None:
+        # TODO: implement with redis-py async
+        pass
+
+    async def delete(self, key: str) -> None:
+        # TODO: implement with redis-py async
+        pass