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/services/coupons.py

@@ -0,0 +1,76 @@
+"""Coupon service — browse coupons, find relevant ones."""
+
+from datetime import date
+from uuid import UUID
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
+
+
+class CouponService:
+    def __init__(self, db: AsyncSession) -> None:
+        self.db = db
+
+    async def list_coupons(self, store_id: UUID | None = None) -> list[dict]:
+        from cartsnitch_api.models import Coupon
+
+        today = date.today()
+        query = (
+            select(Coupon)
+            .where((Coupon.valid_to >= today) | (Coupon.valid_to.is_(None)))
+            .options(selectinload(Coupon.store))
+            .order_by(Coupon.valid_to.asc().nullslast())
+        )
+        if store_id:
+            query = query.where(Coupon.store_id == store_id)
+
+        result = await self.db.execute(query)
+        coupons = result.scalars().all()
+        return [self._to_dict(c) for c in coupons]
+
+    async def relevant_coupons(self, user_id: UUID) -> list[dict]:
+        """Coupons for products the user has purchased."""
+        from cartsnitch_api.models import Coupon, PurchaseItem
+
+        today = date.today()
+
+        # Get product IDs from user's purchase history
+        from cartsnitch_api.models import Purchase
+
+        items_result = await self.db.execute(
+            select(PurchaseItem.normalized_product_id)
+            .join(Purchase)
+            .where(
+                Purchase.user_id == user_id,
+                PurchaseItem.normalized_product_id.isnot(None),
+            )
+            .distinct()
+        )
+        product_ids = [row[0] for row in items_result.all()]
+
+        if not product_ids:
+            return []
+
+        result = await self.db.execute(
+            select(Coupon)
+            .where(
+                Coupon.normalized_product_id.in_(product_ids),
+                (Coupon.valid_to >= today) | (Coupon.valid_to.is_(None)),
+            )
+            .options(selectinload(Coupon.store))
+        )
+        coupons = result.scalars().all()
+        return [self._to_dict(c) for c in coupons]
+
+    def _to_dict(self, c) -> dict:
+        return {
+            "id": c.id,
+            "store_id": c.store_id,
+            "store_name": c.store.name,
+            "description": c.description or c.title,
+            "discount_value": float(c.discount_value) if c.discount_value else 0,
+            "discount_type": c.discount_type,
+            "product_id": c.normalized_product_id,
+            "expires_at": c.valid_to,
+        }