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/tasks/list.json

@@ -12,8 +12,8 @@
     "assignee_ids": [],
     "comments_count": 0,
     "update_line": "",
-    "updated_at": "2026-01-14T10:00:00Z",
-    "created_at": "2026-01-10T09:00:00Z"
+    "updated_at": "2026/01/14 10:00:00 +0000",
+    "created_at": "2026/01/10 09:00:00 +0000"
   },
   {
     "id": 102,
@@ -28,7 +28,7 @@
     "assignee_ids": [1, 2],
     "comments_count": 3,
     "update_line": "",
-    "updated_at": "2026-01-14T11:00:00Z",
-    "created_at": "2026-01-11T10:00:00Z"
+    "updated_at": "2026/01/14 11:00:00 +0000",
+    "created_at": "2026/01/11 10:00:00 +0000"
   }
 ]

testdata/tasks/single.json

@@ -11,6 +11,6 @@
   "assignee_ids": [],
   "comments_count": 0,
   "update_line": "",
-  "updated_at": "2026-01-14T10:00:00Z",
-  "created_at": "2026-01-10T09:00:00Z"
+  "updated_at": "2026/01/14 10:00:00 +0000",
+  "created_at": "2026/01/10 09:00:00 +0000"
 }