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

models_test.go

@@ -0,0 +1,166 @@
+package checkvist
+
+import (
+	"encoding/json"
+	"testing"
+	"time"
+)
+
+func TestAPITime_UnmarshalJSON(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		wantErr bool
+		check   func(t *testing.T, got APITime)
+	}{
+		{
+			name:    "Checkvist API format",
+			input:   `"2026/01/14 16:07:31 +0000"`,
+			wantErr: false,
+			check: func(t *testing.T, got APITime) {
+				if got.Year() != 2026 || got.Month() != 1 || got.Day() != 14 {
+					t.Errorf("date mismatch: got %v", got.Time)
+				}
+				if got.Hour() != 16 || got.Minute() != 7 || got.Second() != 31 {
+					t.Errorf("time mismatch: got %v", got.Time)
+				}
+			},
+		},
+		{
+			name:    "RFC3339 format",
+			input:   `"2026-01-14T10:00:00Z"`,
+			wantErr: false,
+			check: func(t *testing.T, got APITime) {
+				if got.Year() != 2026 || got.Month() != 1 || got.Day() != 14 {
+					t.Errorf("date mismatch: got %v", got.Time)
+				}
+				if got.Hour() != 10 || got.Minute() != 0 {
+					t.Errorf("time mismatch: got %v", got.Time)
+				}
+			},
+		},
+		{
+			name:    "RFC3339 with timezone offset",
+			input:   `"2026-01-14T10:00:00+02:00"`,
+			wantErr: false,
+			check: func(t *testing.T, got APITime) {
+				if got.Year() != 2026 || got.Month() != 1 || got.Day() != 14 {
+					t.Errorf("date mismatch: got %v", got.Time)
+				}
+			},
+		},
+		{
+			name:    "empty string",
+			input:   `""`,
+			wantErr: false,
+			check: func(t *testing.T, got APITime) {
+				if !got.IsZero() {
+					t.Error("expected zero time for empty string")
+				}
+			},
+		},
+		{
+			name:    "null",
+			input:   `null`,
+			wantErr: false,
+			check: func(t *testing.T, got APITime) {
+				if !got.IsZero() {
+					t.Error("expected zero time for null")
+				}
+			},
+		},
+		{
+			name:    "invalid format",
+			input:   `"not a date"`,
+			wantErr: true,
+			check:   nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var got APITime
+			err := json.Unmarshal([]byte(tt.input), &got)
+
+			if (err != nil) != tt.wantErr {
+				t.Errorf("UnmarshalJSON() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if tt.check != nil {
+				tt.check(t, got)
+			}
+		})
+	}
+}
+
+func TestAPITime_MarshalJSON(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    APITime
+		expected string
+	}{
+		{
+			name:     "normal time",
+			input:    NewAPITime(time.Date(2026, 1, 14, 10, 30, 0, 0, time.UTC)),
+			expected: `"2026-01-14T10:30:00Z"`,
+		},
+		{
+			name:     "zero time",
+			input:    APITime{},
+			expected: `null`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := json.Marshal(tt.input)
+			if err != nil {
+				t.Fatalf("MarshalJSON() error = %v", err)
+			}
+			if string(got) != tt.expected {
+				t.Errorf("MarshalJSON() = %s, want %s", got, tt.expected)
+			}
+		})
+	}
+}
+
+func TestAPITime_InStruct(t *testing.T) {
+	// Test unmarshaling a struct with APITime fields using real API format
+	jsonData := `{
+		"id": 1,
+		"name": "Test Checklist",
+		"updated_at": "2026/01/14 16:07:31 +0000"
+	}`
+
+	type testStruct struct {
+		ID        int     `json:"id"`
+		Name      string  `json:"name"`
+		UpdatedAt APITime `json:"updated_at"`
+	}
+
+	var result testStruct
+	err := json.Unmarshal([]byte(jsonData), &result)
+	if err != nil {
+		t.Fatalf("failed to unmarshal: %v", err)
+	}
+
+	if result.UpdatedAt.Year() != 2026 {
+		t.Errorf("expected year 2026, got %d", result.UpdatedAt.Year())
+	}
+	if result.UpdatedAt.Month() != 1 {
+		t.Errorf("expected month 1, got %d", result.UpdatedAt.Month())
+	}
+	if result.UpdatedAt.Day() != 14 {
+		t.Errorf("expected day 14, got %d", result.UpdatedAt.Day())
+	}
+}
+
+func TestNewAPITime(t *testing.T) {
+	now := time.Now()
+	apiTime := NewAPITime(now)
+
+	if !apiTime.Time.Equal(now) {
+		t.Errorf("NewAPITime() did not preserve time: got %v, want %v", apiTime.Time, now)
+	}
+}