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

notes_test.go

@@ -71,8 +71,8 @@ func TestNotes_Create(t *testing.T) {
 				ID:        600,
 				TaskID:    101,
 				Comment:   req.Comment,
-				CreatedAt: time.Now(),
-				UpdatedAt: time.Now(),
+				CreatedAt: NewAPITime(time.Now()),
+				UpdatedAt: NewAPITime(time.Now()),
 			}
 			json.NewEncoder(w).Encode(response)
 		default:
@@ -119,7 +119,7 @@ func TestNotes_Update(t *testing.T) {
 				ID:        501,
 				TaskID:    101,
 				Comment:   req.Comment,
-				UpdatedAt: time.Now(),
+				UpdatedAt: NewAPITime(time.Now()),
 			}
 			json.NewEncoder(w).Encode(response)
 		default: