Implement Timestamp type for Unix timestamp JSON handling

Add custom Timestamp type that wraps time.Time for Kanboard's
Unix timestamp format:

UnmarshalJSON supports:
- Unix timestamps as integers
- Empty strings and "0" strings as zero time
- Null values as zero time
- Numeric strings (e.g., "1609459200")

MarshalJSON returns:
- 0 for zero time
- Unix timestamp for non-zero time

Includes comprehensive tests for round-trip marshaling
and struct embedding scenarios.

Closes: kanboard-api-25y

timestamp.go

@@ -0,0 +1,64 @@
+package kanboard
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+// Timestamp wraps time.Time and handles Kanboard's Unix timestamp JSON format.
+// Kanboard returns timestamps as Unix integers, with 0 or empty strings for null values.
+type Timestamp struct {
+	time.Time
+}
+
+// UnmarshalJSON implements json.Unmarshaler.
+// Supports Unix timestamps as integers, empty strings, "0" strings, and zero values.
+func (t *Timestamp) UnmarshalJSON(data []byte) error {
+	// Try to unmarshal as integer (Unix timestamp)
+	var unix int64
+	if err := json.Unmarshal(data, &unix); err == nil {
+		if unix == 0 {
+			t.Time = time.Time{}
+		} else {
+			t.Time = time.Unix(unix, 0)
+		}
+		return nil
+	}
+
+	// Try to unmarshal as string (empty or "0")
+	var str string
+	if err := json.Unmarshal(data, &str); err == nil {
+		if str == "" || str == "0" {
+			t.Time = time.Time{}
+			return nil
+		}
+		// Try to parse as numeric string
+		var unix int64
+		if _, err := fmt.Sscanf(str, "%d", &unix); err == nil {
+			if unix == 0 {
+				t.Time = time.Time{}
+			} else {
+				t.Time = time.Unix(unix, 0)
+			}
+			return nil
+		}
+	}
+
+	// Handle null
+	if string(data) == "null" {
+		t.Time = time.Time{}
+		return nil
+	}
+
+	return fmt.Errorf("cannot unmarshal timestamp: %s", string(data))
+}
+
+// MarshalJSON implements json.Marshaler.
+// Returns 0 for zero time, otherwise returns Unix timestamp.
+func (t Timestamp) MarshalJSON() ([]byte, error) {
+	if t.IsZero() {
+		return []byte("0"), nil
+	}
+	return json.Marshal(t.Unix())
+}