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)]
+}