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

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: Paperclip <noreply@paperclip.ing>

api/src/cartsnitch_api/routes/coupons.py

@@ -16,7 +16,7 @@ router = APIRouter(prefix="/coupons", tags=["coupons"])
 @router.get("", response_model=list[CouponResponse])
 async def list_coupons(
     store_id: UUID | None = Query(None),
-    user_id: UUID = Depends(get_current_user),
+    user_id: str = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
     svc = CouponService(db)
@@ -25,7 +25,7 @@ async def list_coupons(
 
 @router.get("/relevant", response_model=list[CouponResponse])
 async def relevant_coupons(
-    user_id: UUID = Depends(get_current_user),
+    user_id: str = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
     svc = CouponService(db)