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/single.json

@@ -7,5 +7,5 @@
   "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"
 }