feat: add due calculation

lib/putzplan/tasks/task.ex

@@ -8,13 +8,30 @@ defmodule Putzplan.Tasks.Task do
     defaults [:read, create: :*]
   end
 
+  calculations do
+    calculate :due,
+              :date,
+              expr(
+                fragment(
+                  "SELECT strftime('%F', ?)", # HACK: to cast to date
+                  date_add(
+                    completed_tasks.completion || today(),
+                    repetition_days,
+                    :day
+                  )
+                )
+              )
+  end
+
   sqlite do
     table "tasks"
     repo Putzplan.Repo
   end
 
   relationships do
-    has_many :completed_tasks, Putzplan.Tasks.CompletedTask
+    has_many :completed_tasks, Putzplan.Tasks.CompletedTask do
+      sort [:completion]
+    end
   end
 
   # Attributes are the simple pieces of data that exist on your resource