feat(storage): add S3 backend support and advanced upload limits

- Introduce S3-compatible storage backend support using minio-go.
- Add configuration options for local storage limits, box limits, and rate limiting.
- Implement storage backend selection (local vs S3) for anonymous and registered users.
- Add an `/admin/storage` management interface.
- Update documentation and environment examples with the new configuration variables.

backend/libs/handlers/download.go

@@ -1,8 +1,10 @@
 package handlers
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
+	"io"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -143,12 +145,15 @@ func (a *App) Thumbnail(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	path := a.uploadService.ThumbnailPath(box, file)
-	if path == "" {
+	object, err := a.uploadService.OpenThumbnailObject(r.Context(), box, file)
+	if err != nil {
 		http.ServeFile(w, r, filepath.Join(a.cfg.StaticDir, "img", "file-placeholder.webp"))
 		return
 	}
-	http.ServeFile(w, r, path)
+	defer object.Body.Close()
+	w.Header().Set("Content-Type", "image/jpeg")
+	w.Header().Set("Cache-Control", "public, max-age=604800, immutable")
+	http.ServeContent(w, r, file.ID+"-thumbnail.jpg", object.ModTime, readSeekCloser(object.Body))
 }
 
 func (a *App) UnlockBox(w http.ResponseWriter, r *http.Request) {
@@ -199,31 +204,40 @@ func (a *App) loadFileForRequest(w http.ResponseWriter, r *http.Request) (servic
 }
 
 func (a *App) serveFileContent(w http.ResponseWriter, r *http.Request, box services.Box, file services.File, attachment bool) {
-	path := a.uploadService.FilePath(box, file)
-	source, err := os.Open(path)
-	if err != nil {
-		http.NotFound(w, r)
-		return
-	}
-	defer source.Close()
-
-	stat, err := source.Stat()
+	object, err := a.uploadService.OpenFileObject(r.Context(), box, file)
 	if err != nil {
 		http.NotFound(w, r)
 		return
 	}
+	defer object.Body.Close()
 
 	w.Header().Set("Content-Type", file.ContentType)
 	if attachment {
 		w.Header().Set("Content-Disposition", fmt.Sprintf("attachment; filename=%q", file.Name))
 	}
-	http.ServeContent(w, r, file.Name, stat.ModTime(), source)
+	if seeker, ok := object.Body.(io.ReadSeeker); ok {
+		http.ServeContent(w, r, file.Name, object.ModTime, seeker)
+	} else {
+		if object.Size > 0 {
+			w.Header().Set("Content-Length", fmt.Sprintf("%d", object.Size))
+		}
+		w.WriteHeader(http.StatusOK)
+		_, _ = io.Copy(w, object.Body)
+	}
 
 	if err := a.uploadService.RecordDownload(box.ID); err != nil && !errors.Is(err, os.ErrNotExist) {
 		a.logger.Warn("failed to record file download", "source", "download", "severity", "warn", "code", 4002, "box_id", box.ID, "error", err.Error())
 	}
 }
 
+func readSeekCloser(source io.ReadCloser) io.ReadSeeker {
+	data, err := io.ReadAll(source)
+	if err != nil {
+		return bytes.NewReader(nil)
+	}
+	return bytes.NewReader(data)
+}
+
 func (a *App) DownloadZip(w http.ResponseWriter, r *http.Request) {
 	box, err := a.uploadService.GetBox(r.PathValue("boxID"))
 	if err != nil {