feat: organisation logo (attachOne logo) and payload helpers

- Organisation gains the public attachOne logo (OrganisationLogoField),
  editable as an image upload on the Organisations form; avatar unchanged
- OrganisationLogoURL and OrganisationLogoFields give applications the
  has_logo, logo and logo_url keys for their organisation payloads
- no migration: attachments live in system_files

classes/organisation_logo.go

@@ -0,0 +1,90 @@
+package classes
+
+import (
+	"context"
+	"errors"
+	"strconv"
+
+	"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"
+)
+
+// OrganisationLogoSize is the edge in pixels of the logo thumbnail
+// OrganisationLogoFields answers.
+const OrganisationLogoSize = 256
+
+// organisationLogo loads the organisation's logo attachment, or nil.
+func organisationLogo(ctx context.Context, app *backpack.App, org *models.Organisation) (*attach.File, error) {
+	if app == nil || org == nil || org.ID == 0 {
+		return nil, nil
+	}
+	gdb, ok := app.Lookup[*gorm.DB]()
+	if !ok || gdb == nil {
+		return nil, nil
+	}
+	var f attach.File
+	err := gdb.WithContext(ctx).
+		Where("attachment_type = ? AND attachment_id = ? AND field = ?", org.MorphName(), strconv.FormatUint(uint64(org.ID), 10), models.OrganisationLogoField).
+		Take(&f).Error
+	if errors.Is(err, gorm.ErrRecordNotFound) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &f, nil
+}
+
+// OrganisationLogoURL returns the URL of the organisation's logo at size
+// pixels, the way AvatarURL serves a user's picture: the thumbnail (size by
+// size, mode auto) of the logo attachment, or the attachment's file path
+// when the thumbnail fails (logged). It answers false when the organisation
+// has no logo or no file bucket is published. The URL may be relative; a
+// caller that needs an absolute URL resolves it. Only a failing database
+// lookup is an error.
+func OrganisationLogoURL(ctx context.Context, app *backpack.App, org *models.Organisation, size int) (string, bool, error) {
+	f, err := organisationLogo(ctx, app, org)
+	if err != nil || f == nil {
+		return "", false, err
+	}
+	bucket, ok := app.Lookup[*blob.Bucket]()
+	if !ok || bucket == nil {
+		return "", false, nil
+	}
+	url, err := f.Thumb(ctx, bucket, size, size, "auto")
+	if err != nil {
+		pluginLog.Error("golem15.user: organisation logo thumb", "err", err)
+		return "/" + attach.BlobKey(f.DiskName), true, nil
+	}
+	return url, true, nil
+}
+
+// OrganisationLogoFields answers the logo keys an application adds to its
+// own organisation payloads, named like the user payload's avatar keys:
+// has_logo (bool), logo (the attachment's disk name, or nil) and logo_url
+// (the OrganisationLogoSize thumbnail URL, or nil). An organisation without
+// a logo answers false, nil and nil. Only a failing database lookup is an
+// error.
+func OrganisationLogoFields(ctx context.Context, app *backpack.App, org *models.Organisation) (map[string]any, error) {
+	out := map[string]any{"has_logo": false, "logo": nil, "logo_url": nil}
+	f, err := organisationLogo(ctx, app, org)
+	if err != nil {
+		return nil, err
+	}
+	if f == nil {
+		return out, nil
+	}
+	out["has_logo"] = true
+	out["logo"] = f.DiskName
+	url, ok, err := OrganisationLogoURL(ctx, app, org, OrganisationLogoSize)
+	if err != nil {
+		return nil, err
+	}
+	if ok && url != "" {
+		out["logo_url"] = url
+	}
+	return out, nil
+}