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
