feat: export AvatarURL for the display picture at any size

- AvatarURL returns the avatar thumbnail, the file path when the thumbnail
  fails, or the gravatar URL, the rule of the user payload
- applyAvatar takes avatar_url from it; the payload is unchanged

classes/avatar.go

@@ -0,0 +1,65 @@
+package classes
+
+import (
+	"context"
+	"crypto/md5"
+	"errors"
+	"fmt"
+	"log/slog"
+	"strconv"
+	"strings"
+
+	"git.golem15.com/golem15/sm-user-plugin/models"
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/lagoon/attach"
+	"gocloud.dev/blob"
+	"gorm.io/gorm"
+)
+
+// AvatarURL returns the display picture URL of user at size pixels, the
+// rule the user API payload follows: the thumbnail (size by size, mode
+// auto) of the user's avatar attachment when there is one and a file
+// bucket is published, the attachment's file path when the thumbnail
+// fails (logged), and otherwise the gravatar URL
+// //www.gravatar.com/avatar/{md5 of the email}?s={size}&d=mm. The URL may
+// be protocol-relative or relative; a caller that needs an absolute URL
+// resolves it. Only a failing database lookup is an error.
+func AvatarURL(ctx context.Context, app *backpack.App, user *models.User, size int) (string, error) {
+	if user == nil {
+		return "", errors.New("golem15.user: avatar needs a user")
+	}
+	fallback := gravatar(user.Email, size)
+	if app == nil || user.ID == 0 {
+		return fallback, nil
+	}
+	gdb, ok := app.Lookup[*gorm.DB]()
+	if !ok || gdb == nil {
+		return fallback, nil
+	}
+	var f attach.File
+	err := gdb.WithContext(ctx).
+		Where("attachment_type = ? AND attachment_id = ? AND field = ?", user.MorphName(), strconv.FormatUint(uint64(user.ID), 10), models.AvatarField).
+		Take(&f).Error
+	if errors.Is(err, gorm.ErrRecordNotFound) {
+		return fallback, nil
+	}
+	if err != nil {
+		return "", err
+	}
+	bucket, ok := app.Lookup[*blob.Bucket]()
+	if !ok || bucket == nil {
+		return fallback, nil
+	}
+	url, err := f.Thumb(ctx, bucket, size, size, "auto")
+	if err != nil {
+		slog.Default().Error("golem15.user: avatar thumb", "err", err)
+		return "/" + attach.BlobKey(f.DiskName), nil
+	}
+	return url, nil
+}
+
+// gravatar is the gravatar URL of an email at size pixels.
+func gravatar(email string, size int) string {
+	sum := md5.Sum([]byte(strings.ToLower(strings.TrimSpace(email))))
+	return fmt.Sprintf("//www.gravatar.com/avatar/%x?s=%d&d=mm", sum, size)
+}