feat(backend): implement DiskStorage with on-demand thumbnail/preview cache

Files stored as {files_path}/{id} (no extension). The ext parameter
is removed from Save/Read/Delete in both the port interface and
the implementation.

Thumbnail and Preview both use imaging.Thumbnail (fit within
configured max bounds, never upscale, never crop) — the config
values THUMB_WIDTH/HEIGHT and PREVIEW_WIDTH/HEIGHT are upper limits,
not forced dimensions.

Non-decodable files (video, etc.) receive a #444455 placeholder.
Cache writes use atomic temp→rename; on cache failure the generated
image is served from memory so the request still succeeds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/go.mod

@@ -17,6 +17,7 @@ require (
 	github.com/bytedance/sonic v1.15.0 // indirect
 	github.com/bytedance/sonic/loader v0.5.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
+	github.com/disintegration/imaging v1.6.2 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.12 // indirect
 	github.com/gin-contrib/sse v0.1.0 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
@@ -43,6 +44,7 @@ require (
 	go.uber.org/multierr v1.11.0 // indirect
 	golang.org/x/arch v0.22.0 // indirect
 	golang.org/x/crypto v0.39.0 // indirect
+	golang.org/x/image v0.0.0-20191009234506-e7c1f5e7dbb8 // indirect
 	golang.org/x/net v0.33.0 // indirect
 	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/sys v0.41.0 // indirect