domain: primaryImageTag -> imageId end-to-end (phase 4a/4b)

Rust: PlayerMediaItem and MergedMediaItem gain image_id (dual-carry),
populated from primary_image_tag at every construction/conversion site.
Regenerated bindings.

Frontend: all catalog + player + merged readers now use imageId. The
NowPlayingItem->MediaItem bridge (player.ts) properly maps the remote
session's Jellyfin fields (Type, runTimeTicks, primaryImageTag) onto the
neutral kind/durationMs/imageId. Types that are genuinely out of scope
(Person, NowPlayingItem, PlayItemRequest) keep primaryImageTag.

Rust 456, frontend 644, check clean.

src/lib/components/player/VideoPlayer.svelte

@@ -1223,7 +1223,7 @@
           needsTranscoding: false,
           // Now-playing metadata so the lockscreen/miniplayer show the item.
           artist: media.seriesName ?? null,
-          primaryImageTag: media.primaryImageTag ?? null,
+          primaryImageTag: media.imageId ?? null,
           serverId: media.serverId ?? null,
           // Real duration so the lockscreen scrubber has a range to draw.
           durationSeconds: duration > 0 ? duration : null,
@@ -1626,11 +1626,11 @@
     {#if !isMediaReady}
       <div class="absolute inset-0 flex items-center justify-center bg-black">
         <!-- Poster/Title Card -->
-        {#if media?.primaryImageTag}
+        {#if media?.imageId}
           <CachedImage
             itemId={media.id}
             imageType="Primary"
-            tag={media.primaryImageTag}
+            tag={media.imageId}
             maxHeight={1080}
             alt={media?.name || "Video"}
             class="max-w-full max-h-full object-contain"