feat(70): accept per-source weight on the multi-collection API

Without this the weight is only reachable by editing the database, so the
enumerator has nothing to distribute by.

Weight is threaded through create and update (all four handler branches: add and
update, plain and smart) and defaults to 1, so it is optional on the wire and
/api/v1 stays additive under the freeze.

It is returned on the read path too, which is load-bearing rather than symmetry:
the update replaces the item list, so a client that GETs, edits a name, and PUTs
back would silently reset every weight to the default if the GET didn't carry it.

Weight edits ride the existing MultiCollection Version token, so If-Match/412
concurrency needs no new design.

Regenerated v1.json + v1.d.ts + endpoint-index via update-openapi.sh and
generate:api (never hand-edited). The spec picks up weight on both request and
response models and WeightedShuffle in the PlaybackOrder enum; weight is emitted
optional.

Refs #70

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

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

@@ -936,6 +936,7 @@ export interface components {
     "smartCollectionId": null | number;
     "scheduleAsGroup": boolean;
     "playbackOrder": components["schemas"]["PlaybackOrder"];
+    "weight"?: number;
   };
     "MultiCollectionItemResponseModel": {
     "collectionId": null | number;
@@ -943,6 +944,7 @@ export interface components {
     "name": string;
     "scheduleAsGroup": boolean;
     "playbackOrder": components["schemas"]["PlaybackOrder"];
+    "weight"?: number;
   };
     "MultiCollectionResponseModel": {
     "id": number;
@@ -998,7 +1000,7 @@ export interface components {
     "remotePath": null | string;
     "localPath": null | string;
   };
-    "PlaybackOrder": "None" | "Chronological" | "Random" | "Shuffle" | "ShuffleInOrder" | "MultiEpisodeShuffle" | "SeasonEpisode" | "RandomRotation" | "Marathon";
+    "PlaybackOrder": "None" | "Chronological" | "Random" | "Shuffle" | "ShuffleInOrder" | "MultiEpisodeShuffle" | "SeasonEpisode" | "RandomRotation" | "Marathon" | "WeightedShuffle";
     "PlaylistGroupResponseModel": {
     "id": number;
     "name": string;