Auto-create books directory, log upload errors, document Docker deployment

- library.New now creates BOOKS_DIR (MkdirAll) on startup so uploads
  don't fail when the directory is missing on a fresh bind mount
- uploadSubmit logs the underlying OS error on save/write failures
- README: document docker-compose deployment, container UID/GID (100/101)
  for bind-mounted ./data permissions, and running the admin CLI via
  docker compose exec

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/web/handlers.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -232,12 +233,14 @@ func (h *Handler) uploadSubmit(w http.ResponseWriter, r *http.Request) {
 			render(w, r.Context(), views.UploadPage(fmt.Sprintf("Datei '%s' existiert bereits.", safeFilename)))
 			return
 		}
+		log.Printf("upload: create %q failed: %v", destPath, err)
 		http.Error(w, "Datei konnte nicht gespeichert werden", http.StatusInternalServerError)
 		return
 	}
 	defer out.Close()
 
 	if _, err := io.Copy(out, file); err != nil {
+		log.Printf("upload: write %q failed: %v", destPath, err)
 		_ = os.Remove(destPath)
 		http.Error(w, "Upload fehlgeschlagen", http.StatusInternalServerError)
 		return