fix(api): replace UUID type with str for Better-Auth nanoid user IDs (#98)

Better-Auth uses nanoid strings for user IDs, not UUIDs. Changed all
user_id parameter/return types in the API layer from UUID to str,
removed the obsolete UUID import where unused, and updated the
_validate_session_token return type accordingly.

Co-authored-by: CartSnitch Engineer Bot <cartnoreply@cartsnitch.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/services/coupons.py

@@ -29,7 +29,7 @@ class CouponService:
         coupons = result.scalars().all()
         return [self._to_dict(c) for c in coupons]
 
-    async def relevant_coupons(self, user_id: UUID) -> list[dict]:
+    async def relevant_coupons(self, user_id: str) -> list[dict]:
         """Coupons for products the user has purchased."""
         from cartsnitch_api.models import Coupon, PurchaseItem