Make createTransaction's rundown deep-readonly when not mutable

Overload createTransaction() on the literal mutableRundown option: with
mutableRundown: true it returns rundown: Rundown as before; otherwise it
returns rundown: DeepReadonly<Rundown> (ts-essentials, already a convention
in this codebase for read-only snapshots).

Previously a non-mutable transaction's rundown was typed as plain Rundown
even though it's the live cachedRundown reference itself (or a background
rundown read straight from disk) - nothing stopped a future mutation
function from writing into it outside the commit() flow, since
Readonly<T> (used elsewhere for the same purpose) only blocks top-level
reassignment, not nested writes like array.push() or entry.field = x.

Scoped to rundown.dao.ts only: every existing call site in rundown.service.ts
passes a literal mutableRundown: true, so this changes no call-site types.
The one mutableRundown: false site doesn't destructure rundown at all.
Verified with a throwaway probe file (removed) that mutating a non-mutable
transaction's rundown is now a compile error, and that a mutable one still
compiles as before. Typecheck, lint and full test suite (695 tests) pass
unchanged.

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

@@ -27,6 +27,7 @@ import {
   isPlayableEvent,
 } from 'ontime-types';
 import { addToRundown, createGroup, customFieldLabelToKey, getInsertAfterId, insertAtIndex } from 'ontime-utils';
+import type { DeepReadonly } from 'ts-essentials';
 
 import { getDataProvider } from '../../classes/data-provider/DataProvider.js';
 import { consoleError } from '../../utils/console.js';
@@ -80,9 +81,16 @@ export const getRundownMetadata = (): Readonly<RundownMetadata> => rundownMetada
 export const getProjectCustomFields = (): Readonly<CustomFields> => projectCustomFields;
 export const getEntryWithId = (entryId: EntryId): OntimeEntry | undefined => cachedRundown.entries[entryId];
 
-type Transaction = {
+/**
+ * @param R the type callers see for `rundown` - a plain, mutable `Rundown` when the
+ * transaction was opened with `mutableRundown: true`, otherwise a `DeepReadonly<Rundown>`
+ * so that accidentally mutating an entry (or an order array) on a non-mutable transaction
+ * - which would silently corrupt the live cache without going through commit() - is a
+ * compile-time error instead of a runtime bug.
+ */
+type Transaction<R> = {
   customFields: CustomFields;
-  rundown: Rundown;
+  rundown: R;
 
   commit: (shouldProcess?: boolean) => Promise<{
     rundown: Readonly<Rundown>;
@@ -103,7 +111,13 @@ type TransactionOptions = {
   rundownId?: string;
 };
 
-export function createTransaction(options: TransactionOptions): Transaction {
+export function createTransaction(options: TransactionOptions & { mutableRundown: true }): Transaction<Rundown>;
+export function createTransaction(
+  options: TransactionOptions & { mutableRundown?: false },
+): Transaction<DeepReadonly<Rundown>>;
+export function createTransaction(
+  options: TransactionOptions,
+): Transaction<Rundown> | Transaction<DeepReadonly<Rundown>> {
   const targetId = options.rundownId ?? cachedRundown.id;
   const isLoaded = targetId === cachedRundown.id;
   const sourceRundown: Rundown = isLoaded ? cachedRundown : (getDataProvider().getRundown(targetId) as Rundown);