feat(#259): id-based reconcile for schedule-items replace (backend + DTO)

Add optional `int? Id` to ScheduleItemRequest/ReplaceProgramScheduleItem so
a client can round-trip each existing item's server id. When ids are present,
ReplaceProgramScheduleItemsHandler reconciles by id (not array position), so an
item's persisted fill-group/shuffle state (PlayoutScheduleItemFillGroupIndex,
FK OnDelete Cascade) follows the logical item across reorders/inserts instead of
being inherited by whatever previously occupied its new slot (#259, split from
#252/#253). A fully id-less payload keeps the verbatim positional fallback.

Guards (inside PersistItems, after CheckVersion so 412 precedes 422): duplicate
id -> 422; id not in this schedule -> 422 (a stale id under Phase-1 force-write is
a live lost-update signal, not a new item). Index stays array-position derived.

Regenerated v1.json + TS client.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/api/generated/v1.d.ts

@@ -1279,6 +1279,7 @@ export interface components {
     "deepScan"?: boolean;
   };
     "ScheduleItemRequest": {
+    "id": null | number;
     "startType": components["schemas"]["StartType"];
     "startTime": null | string;
     "fixedStartTimeBehavior": null | components["schemas"]["FixedStartTimeBehavior"];