feat(19.3-03): storage.uploads.public_url for direct bucket or CDN file URLs

- OpenBucket validates public_url (absolute http/https, host, no user info, query
  or fragment; the error names the key) and records it next to public_path_prefix
- a set public_url logs one start-up line: the bucket behind it must hold only
  public files, since it bypasses the is_public check
- attach.PublicURLBase; PublicURL, File.URL and thumbnail URLs use the base when
  set and keep the public_path_prefix proxy URLs otherwise
- TestPublicURLSmoke; storage Public URL base section, attachments, configuration
  and lagoon README

modules/lagoon/README.md

@@ -38,7 +38,7 @@ Use cases:
 - Lifecycle and relations: hook interfaces matching GORM's native method names (`lagoon.HasBeforeCreate`, `lagoon.HasBeforeSave`, `lagoon.HasBeforeDelete`, `lagoon.HasAfterDelete`) plus `lagoon.HasBeforeValidate`; `lagoon.WithSoftDeleteCascade` runs a cascade inside the parent delete; `lagoon.RegisterJoinTable` wires pivot models with business columns.
 - Deferred binding: WinterCMS's `deferred_bindings` table holds the uploads and related-record changes of a form whose record is not saved yet. Every operation takes a `lagoon.DeferredKey` (the form's session key, the owning backend admin's id and the master record's morph type from `lagoon.MorphType`) and never reads or changes another admin's rows, since each row stores `backend_user_id`. `lagoon.DeferredBind` and `lagoon.DeferredUnbind` port WinterCMS's duplicate and cancel rules: a repeated bind writes nothing, and an unbind of a slave with a pending bind deletes that bind and returns it so the caller can remove what it created. `lagoon.DeferredBindings` reads and locks a session's bindings for the save that commits them, `lagoon.DeferredForget` deletes them once applied, and `lagoon.DeferredSlaves` is the subquery a list uses to include pending rows. A child created under deferral carries the `lagoon.DeferredEnvelope` (`{"created":true,"pivot":{...},"upload_id":"..."}`) in `pivot_data`. `lagoon.PurgeDeferred` removes expired bindings: it deletes an unattached `system_files` row a bind points at, and its blobs only after the commit, deletes a child only when its binding carries the created envelope, keeps records that were only linked, and locks each batch with `FOR UPDATE SKIP LOCKED`.
 - Imports from Laravel: `lagoon.DecryptLaravelPayload` decrypts Laravel `encrypted` payloads with the old application key, for one-off data imports.
-- Attachments (`attach`): the `attach.File` model for `system_files` rows, WinterCMS-compatible partitioned storage keys (`attach.BlobKey`, `attach.PartitionDirectory`), an S3 bucket option behind the `attach.S3BuildTag` build tag (an untagged binary refuses an `s3://` bucket URL at boot with an error naming the tag), public URLs (`attach.PublicURL` for any key, `attach.File.URL` for an original, matching WinterCMS's `File::getPath()` under the WinterCMS layout), on-demand thumbnails through `attach.File.Thumb` for JPEG, PNG, GIF and WebP originals (a WebP original's thumbnail is JPEG bytes under its `.webp` name, since WebP cannot be encoded; a missing, undecodable or oversized original gets WinterCMS's broken-image picture, `attach.BrokenImagePNG`, as its thumbnail, as `File::makeThumb` does), storing uploads through `attach.Store` (a server-generated disk name whose extension follows the sniffed content through `attach.DiskExtension`, so a PNG named `x.html` is stored as `.png`, an extension allow-list with `attach.DefaultImageExtensions` and `attach.DefaultFileExtensions` as defaults, a MIME filter, a size limit enforced while streaming and, in image mode, the `attach.IsAllowedImage` content guard), attachment relation declarations (`attach.Relation`, `attach.HasRelations`), static serving with an optional `is_public` gate (`attach.StaticHandlerPublic`), and a two-phase delete that removes blobs only after the database transaction commits (`attach.DeleteForOwner`, `attach.DeleteKeys`).
+- Attachments (`attach`): the `attach.File` model for `system_files` rows, WinterCMS-compatible partitioned storage keys (`attach.BlobKey`, `attach.PartitionDirectory`), an S3 bucket option behind the `attach.S3BuildTag` build tag (an untagged binary refuses an `s3://` bucket URL at boot with an error naming the tag), public URLs (`attach.PublicURL` for any key, under `storage.uploads.public_url` when a public bucket or CDN serves the files (`attach.PublicURLBase`), `attach.File.URL` for an original, matching WinterCMS's `File::getPath()` under the WinterCMS layout), on-demand thumbnails through `attach.File.Thumb` for JPEG, PNG, GIF and WebP originals (a WebP original's thumbnail is JPEG bytes under its `.webp` name, since WebP cannot be encoded; a missing, undecodable or oversized original gets WinterCMS's broken-image picture, `attach.BrokenImagePNG`, as its thumbnail, as `File::makeThumb` does), storing uploads through `attach.Store` (a server-generated disk name whose extension follows the sniffed content through `attach.DiskExtension`, so a PNG named `x.html` is stored as `.png`, an extension allow-list with `attach.DefaultImageExtensions` and `attach.DefaultFileExtensions` as defaults, a MIME filter, a size limit enforced while streaming and, in image mode, the `attach.IsAllowedImage` content guard), attachment relation declarations (`attach.Relation`, `attach.HasRelations`), static serving with an optional `is_public` gate (`attach.StaticHandlerPublic`), and a two-phase delete that removes blobs only after the database transaction commits (`attach.DeleteForOwner`, `attach.DeleteKeys`).
 
 ## Usage
 
@@ -191,7 +191,8 @@ func (p *Plugin) Migrations() []*gormigrate.Migration {
 | `lagoon.RegisterJoinTable` | Registers a custom pivot model for a many-to-many field. |
 | `lagoon.DecryptLaravelPayload` | Decrypts a Laravel AES-256-CBC payload for data imports. |
 | `attach.File` | The `system_files` row model; `attach.File.URL` is the public URL of the original. |
-| `attach.PublicURL` | Public URL of a blob key under `storage.uploads.public_path_prefix`. |
+| `attach.PublicURL` | Public URL of a blob key: under `storage.uploads.public_url` when set, else under `storage.uploads.public_path_prefix`. |
+| `attach.PublicURLBase` | `storage.uploads.public_url` as recorded by `attach.OpenBucket`, empty when unset. |
 | `attach.BrokenImagePNG` | WinterCMS's broken-image picture, stored as the thumbnail of an unusable original. |
 | `attach.Owner` | Implemented by models that own attachments; returns the stored morph type name. |
 | `attach.OpenBucket` | Opens the uploads bucket from config; refuses `s3://` in a binary built without `attach.S3BuildTag`. |
@@ -231,6 +232,7 @@ Keys are read from the compass config; the `SUMMER_` environment overlay maps a
 | `app.previous_keys` | empty | List of earlier base64 keys still accepted when decrypting, for key rotation. |
 | `storage.uploads.bucket_url` | none (required by `attach.OpenBucket`) | Uploads bucket URL, `file://`, `mem://` or `s3://` (with `attach.S3BuildTag`; query parameters such as `region`, `endpoint` and `use_path_style` per gocloud.dev, credentials from the AWS environment). |
 | `storage.uploads.public_path_prefix` | `/storage/uploads` | URL prefix used when building public file and thumbnail URLs. |
+| `storage.uploads.public_url` | empty | Absolute `http` or `https` base of a public bucket or CDN; when set, file and thumbnail URLs are this base plus the partition path. A malformed value fails `attach.OpenBucket`; a set value logs that the bucket must hold only public files. |
 | `database.deferred_bindings.purge_days` | `5` | Age in days after which `deferred:purge` removes a deferred binding, WinterCMS's `cleanUp(5)`; the `--days` flag overrides it. |
 | `database.deferred_bindings.purge_at` | `03:00` | Daily `HH:MM` (in `app.timezone`) at which the scheduler runs `deferred:purge`; an empty value disables the framework entry. |