fix(12-05): store WinterCMS's broken-image thumbnail for an unusable original (T-12-16)

A photo whose original is missing, does not decode or declares more than
4096x4096 pixels made attach.File.Thumb return an error, and every listing
that shows the photo answered 500 from then on: one 100-byte PNG uploaded
by any household member broke GET collections and the album for everyone.

Thumb now follows WinterCMS's File::makeThumb catch branch: it logs the
reason at warn level, stores WinterCMS's BrokenImage picture (exported as
attach.BrokenImagePNG) under the thumbnail key and returns its URL. Invalid
arguments, storage errors and encode failures are still errors.

modules/lagoon/README.md

@@ -24,7 +24,7 @@ Postgres data layer: the shared GORM connection, per-plugin migrations, model he
 - Column types: `lagoon.Encrypted` stores AES-256-GCM ciphertext under a key derived from `app.key`, decrypts with previous keys during rotation, and always redacts itself in JSON and string output; `lagoon.Jsonable` stores JSON as TEXT and keeps SQL NULL distinct from an empty value.
 - 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.
 - 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`), 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), 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`), 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), 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
 
@@ -153,6 +153,7 @@ func (p *Plugin) Migrations() []*gormigrate.Migration {
 | `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.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. |
 | `attach.Publish` | Stores the bucket on the `backpack.App`. |