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>

api/src/cartsnitch_api/services/queries.py

@@ -0,0 +1,23 @@
+"""Shared query helpers for service layer."""
+
+from uuid import UUID
+
+from sqlalchemy import func, select
+
+
+def latest_price_per_store(product_ids: list[UUID] | None = None):
+    """Subquery returning the latest observed_date per product+store.
+
+    Optionally filtered to a list of product IDs. Returns a subquery with
+    columns: normalized_product_id, store_id, max_date.
+    """
+    from cartsnitch_api.models import PriceHistory
+
+    query = select(
+        PriceHistory.normalized_product_id,
+        PriceHistory.store_id,
+        func.max(PriceHistory.observed_date).label("max_date"),
+    ).group_by(PriceHistory.normalized_product_id, PriceHistory.store_id)
+    if product_ids is not None:
+        query = query.where(PriceHistory.normalized_product_id.in_(product_ids))
+    return query.subquery()