feat: optional S3-compatible storage for uploaded images, instead of the local upload directory

docs/architecture.md

@@ -57,7 +57,7 @@ The use cases `GetPurgeSettings`, `UpdatePurgeSettings`, `PreviewPurge` and `Run
 
 ### Image metadata backfill
 
-At boot, `BackfillImageMetadataUseCase` (through `image.Repository.ListMissingMetadata` and `UpdateMetadata`) fills `filename`, `size`, `content_type`, `width`, `height` and `sha256` for image rows that lack them, reading the bytes through `image.Storage` (the files in `UPLOAD_DIR` with the local adapter). It is idempotent, and a row whose file is missing or unreadable is left as it is without stopping the boot. The old `article_id` column and the `article_images` table are kept and never written.
+At boot, `BackfillImageMetadataUseCase` (through `image.Repository.ListMissingMetadata` and `UpdateMetadata`) fills `filename`, `size`, `content_type`, `width`, `height` and `sha256` for image rows that lack them, reading the bytes through `image.Storage` (the files in `UPLOAD_DIR` with the local adapter, the bucket objects with the S3 adapter). It is idempotent, and a row whose file is missing or unreadable is left as it is without stopping the boot. The old `article_id` column and the `article_images` table are kept and never written.
 
 ## Frontend ↔ backend
 
@@ -94,4 +94,15 @@ Backups are a `domain/update.DatabaseBackup` per driver. SQLite uses `VACUUM INT
 
 `task test` runs everything that needs no server. The repository contract tests (`test/repository_contract_test.go`) run on SQLite always, and on MySQL when `RENEWCMS_TEST_MYSQL_DSN` points at a server (admin DSN; each test creates and drops its own database). `task test:mysql` starts `mysql:8.4` and then `mariadb:11` in Docker and runs the `MySQL` tests against each.
 
+Two storage adapters implement `image.Storage`: `local` (default, files in `UPLOAD_DIR`) and `s3` (`storage.S3Storage`, `aws-sdk-go-v2` over plain HTTPS, pure Go so `CGO_ENABLED=0` still holds). `STORAGE_DRIVER` picks one in `imageStorageConfig` (composition root, from `config.StorageSettings`); `storage.Open` fails with `unsupported storage driver "<name>"` for anything else, and `openImageStorage` panics with the error. The S3 adapter works as follows, with no change to the domain, use cases, repositories or the `staticimages` handler:
+
+- `Open` calls `GetObject` and returns a `Blob` whose `Content` is an `io.ReadSeeker` (`s3Reader`): `Size` and `ModTime` come from `ContentLength` and `LastModified`, the first bytes are read from that response and every `Seek` to another offset makes the next `Read` issue a ranged `GetObject`. `http.ServeContent` therefore answers `Range` (206, 416) and `If-Modified-Since` (304) with no `ETag`. A 404 maps to `image.ErrBlobNotFound`; any other failure is a plain error (500 in the handler).
+- `Put` does one `PutObject` of `<S3_KEY_PREFIX><uuid>.<ext>` with the upload's `Content-Type`. `Delete` treats 404 as success. Names that are not plain file names are never requested, as in the local adapter.
+- Each call has a 30 second timeout to get a response (the body then streams without a deadline) and the SDK default retries. Request checksums are only sent when required, so older S3-compatible servers work.
+- `storage.Open` checks the bucket with `HeadBucket`. The failure text names the bucket and the reason (404, 403, other status or transport error) and never the credentials.
+- At boot, `RunBackfillReportingReads` wraps the storage in a counter for the `BackfillImageMetadataUseCase` run and logs how many images it read; the use case itself is unchanged.
+- Update backups (`DATA_DIR`) never include uploads, whatever the driver.
+
+`task test:s3` starts SeaweedFS in Docker and runs `TestS3StorageIntegration` against it (skipped without Docker, and not part of `task test`). Everything else uses an in-process fake S3 server listening on `127.0.0.1`.
+
 Adding a database driver: implement `database.Driver` (`Open`, and `NewBackup` returning a `domain/update.DatabaseBackup`), register it in `drivers`, then select it in `databaseConfig` in the composition root (and add the env vars to `main.go`, `.env.example` and `configuration.md`). Adding a storage backend: implement `image.Storage`, add a case to `storage.Open`, and select it in `openImageStorage`. No use case, handler or repository changes.