fix(library): reject reserved names (cache, .uploads) with 400 reserved_name (B8)

backend/cmd/webui/handlers/libraries.go

@@ -15,6 +15,7 @@ import (
 	"github.com/jackc/pgx/v5"
 
 	"booklib/internal/bookfile"
+	"booklib/internal/media"
 	"booklib/internal/store"
 )
 
@@ -61,6 +62,11 @@ func (h *H) CreateLibrary(c *gin.Context) {
 		err(c, http.StatusBadRequest, "bad_request", "bad name")
 		return
 	}
+	// B8: reject reserved names that conflict with system directories.
+	if media.IsReservedName(safe) {
+		err(c, http.StatusBadRequest, "bad_request", "reserved_name")
+		return
+	}
 	root := filepath.Join(filepath.Clean(h.cfg.BooksDir), safe)
 	id, e := h.st.CreateLibrary(c, req.Name, root)
 	if e != nil {

backend/internal/media/reserved.go

@@ -0,0 +1,16 @@
+package media
+
+import "strings"
+
+// reservedNames are library names that conflict with system directories.
+// This is the single source of truth for reserved name validation (B8).
+var reservedNames = map[string]bool{
+	"cache":    true, // CACHE_DIR
+	".uploads": true, // upload session directory
+	".trash":   true, // potential future use
+}
+
+// IsReservedName reports whether name conflicts with system directories.
+func IsReservedName(name string) bool {
+	return reservedNames[strings.ToLower(name)]
+}

backend/internal/media/reserved_test.go

@@ -0,0 +1,16 @@
+package media
+
+import "testing"
+
+func TestIsReservedName(t *testing.T) {
+	for _, n := range []string{"cache", "Cache", "CACHE", ".uploads", ".Uploads", ".trash"} {
+		if !IsReservedName(n) {
+			t.Errorf("IsReservedName(%q) = false, want true", n)
+		}
+	}
+	for _, n := range []string{"comics", "books", "my-library", "Cache1"} {
+		if IsReservedName(n) {
+			t.Errorf("IsReservedName(%q) = true, want false", n)
+		}
+	}
+}