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

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)
+		}
+	}
+}