feat(store): add recur and recur_mode task fields

Problem: the task schema has no fields for storing recurrence rules.

Solution: add recur and recur_mode to the Task class, known_fields,
task_to_table, table_to_task, and the add() signature.

spec/store_spec.lua

@@ -196,6 +196,41 @@ describe('store', function()
     end)
   end)
 
+  describe('recurrence fields', function()
+    it('persists recur and recur_mode through round-trip', function()
+      store.load()
+      store.add({ description = 'Recurring', recur = 'weekly', recur_mode = 'scheduled' })
+      store.save()
+      store.unload()
+      store.load()
+      local task = store.get(1)
+      assert.are.equal('weekly', task.recur)
+      assert.are.equal('scheduled', task.recur_mode)
+    end)
+
+    it('persists recur without recur_mode', function()
+      store.load()
+      store.add({ description = 'Simple recur', recur = 'daily' })
+      store.save()
+      store.unload()
+      store.load()
+      local task = store.get(1)
+      assert.are.equal('daily', task.recur)
+      assert.is_nil(task.recur_mode)
+    end)
+
+    it('omits recur fields when not set', function()
+      store.load()
+      store.add({ description = 'No recur' })
+      store.save()
+      store.unload()
+      store.load()
+      local task = store.get(1)
+      assert.is_nil(task.recur)
+      assert.is_nil(task.recur_mode)
+    end)
+  end)
+
   describe('active_tasks', function()
     it('excludes deleted tasks', function()
       store.load()