refactor(server): replace defensive copies with cow patterns

apps/server/src/api-data/rundown/rundown.utils.ts

@@ -329,7 +329,7 @@ export function mergeRundownPreservingFields(
     const structure = isOntimeGroup(incomingEntry)
       ? { entries: incomingEntry.entries }
       : { parent: incomingEntry.parent };
-    entries[id] = structuredClone({ ...merged, ...structure });
+    entries[id] = cloneEntryData({ ...merged, ...structure });
   }
 
   return {
@@ -499,6 +499,61 @@ export function cloneSimpleRundownEntry(entry: OntimeEntry, newId: EntryId): Ont
   throw new Error(`Unsupported entry type for cloning: ${entry}`);
 }
 
+/**
+ * Fast, shape-aware clone of a single entry, preserving its identity (id, revision, etc).
+ * Drop-in replacement for `structuredClone(entry)`
+ */
+export function cloneEntryData<T extends OntimeEntry>(entry: T): T {
+  switch (entry.type) {
+    case SupportedEntry.Event: {
+      const clone: OntimeEvent = { ...entry };
+      if (clone.custom) clone.custom = { ...clone.custom };
+      if (clone.triggers) clone.triggers = clone.triggers.map((trigger) => ({ ...trigger }));
+      return clone as T;
+    }
+    case SupportedEntry.Group: {
+      const clone: OntimeGroup = { ...entry };
+      if (clone.custom) clone.custom = { ...clone.custom };
+      if (clone.entries) clone.entries = clone.entries.slice();
+      return clone as T;
+    }
+    case SupportedEntry.Milestone: {
+      const clone: OntimeMilestone = { ...entry };
+      if (clone.custom) clone.custom = { ...clone.custom };
+      return clone as T;
+    }
+    case SupportedEntry.Delay:
+      return { ...entry } as T;
+    default: {
+      // exhaustiveness guard: a new member of `SupportedEntry` is named in the error here
+      const unhandled: never = entry;
+      throw new Error(`Unsupported entry type for cloning: ${(unhandled as OntimeEntry).type}`);
+    }
+  }
+}
+
+/**
+ * Fast, shape-aware clone of a whole rundown.
+ * Drop-in replacement for `structuredClone(rundown)`: every entry (and its nested
+ * `custom` / `triggers` / `entries` containers) gets its own copy, so callers can mutate
+ * the result freely without touching the source - same contract as structuredClone,
+ * at a fraction of the cost since we skip the generic serialization algorithm.
+ */
+export function cloneRundown(rundown: Readonly<Rundown>): Rundown {
+  const entries: RundownEntries = {};
+  for (const id in rundown.entries) {
+    entries[id] = cloneEntryData(rundown.entries[id]);
+  }
+  return {
+    id: rundown.id,
+    title: rundown.title,
+    revision: rundown.revision,
+    order: rundown.order.slice(),
+    flatOrder: rundown.flatOrder.slice(),
+    entries,
+  };
+}
+
 /**
  * Utility for calculating if the current events should have a day offset
  * @param current the current event under test