Fix date parsing for Checkvist API format

Add custom APITime type with UnmarshalJSON to handle the non-standard
date format returned by the Checkvist API ("2026/01/14 16:07:31 +0000")
instead of the expected RFC3339 format.

Changes:
- Add APITime type with custom JSON marshaling/unmarshaling in models.go
- Replace time.Time with APITime for UpdatedAt/CreatedAt fields in
  Checklist, Task, and Note structs
- Add NewAPITime constructor for convenience
- Update test fixtures to use real API date format
- Add comprehensive unit tests for APITime in models_test.go

Fixes: checkvist-api-4qn

testdata/checklists/list.json

@@ -8,7 +8,7 @@
     "task_count": 10,
     "task_completed": 3,
     "tags_as_text": "",
-    "updated_at": "2026-01-14T10:00:00Z"
+    "updated_at": "2026/01/14 10:00:00 +0000"
   },
   {
     "id": 2,
@@ -19,6 +19,6 @@
     "task_count": 25,
     "task_completed": 12,
     "tags_as_text": "work, important",
-    "updated_at": "2026-01-13T15:30:00Z"
+    "updated_at": "2026/01/13 15:30:00 +0000"
   }
 ]