feat(12.2-01): add lagoon.Date and lagoon.TimeOfDay with Fill and required support

- Date (DATE) and TimeOfDay (TIME) with Scanner, Valuer, JSON and text forms
- Fill falls back to encoding.TextUnmarshaler for string sources after
  every existing conversion, so time.Time and the new types fill from JSON
- required treats a zero time.Time, Date or TimeOfDay as empty
- lagoon README, models and casts-and-validation docs

modules/lagoon/validate.go

@@ -9,6 +9,7 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"time"
 
 	"git.golem15.com/golem15/summercms/modules/phrasebook"
 	"github.com/go-playground/validator/v10"
@@ -265,6 +266,22 @@ func isEmptyValue(val any) bool {
 	if val == nil {
 		return true
 	}
+	// A zero time.Time, Date or TimeOfDay is "no value" for required. Only
+	// these three types are checked, so no other type's emptiness changes.
+	switch v := val.(type) {
+	case time.Time:
+		return v.IsZero()
+	case *time.Time:
+		return v == nil || v.IsZero()
+	case Date:
+		return v.IsZero()
+	case *Date:
+		return v == nil || v.IsZero()
+	case TimeOfDay:
+		return v.IsZero()
+	case *TimeOfDay:
+		return v == nil || v.IsZero()
+	}
 	rv := reflect.ValueOf(val)
 	switch rv.Kind() {
 	case reflect.Ptr, reflect.Interface: