feat: image variants cache panel

docs/architecture.md

@@ -75,6 +75,8 @@ Producers: `ArticleChanges`, `TextChange` and `BodyChange` in `internal/domain/a
 
 Generated bytes are stored only through `image.Storage`, so they follow `STORAGE_DRIVER`, and are never rows of `images`. `ServeVariantUseCase` checks, in order: unsupported parameters, the variant (it must exist and be enabled), the source blob, SVG and animated GIF (served as they are), the size cap, the `If-None-Match` ETag, then the cached file, and generates under a `singleflight` keyed by variant, settings and source. A row whose fingerprint no longer matches, or whose blob vanished, is replaced. `UpdateVariantUseCase` purges a variant's files when a rendering setting changes; `DeleteVariantUseCase` and `PurgeVariantUseCase` purge a blob and then its row, so a failed blob delete keeps the row and a retry can finish; `mediaRemover` purges the files of a source before it deletes the source (delete and merge).
 
+The cache screen adds three ports in `internal/domain/image`: `VariantCacheRepository` (unused totals, top sources by bytes and cursor-paged row listing, implemented by the same persistence type as `VariantFileRepository`), `VariantUsageRepository` (one batched write of request times and per-day rejection counts, and the rejection total of a window) and `VariantUsageTracker` (in-memory hits and rejections). `internal/infrastructure/usage.Tracker` implements the tracker; `GetVariantCacheStatsUseCase` and `PurgeVariantCacheUseCase` use the ports only. See [image-variants.md](image-variants.md).
+
 ### 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, 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.