feature: Phase 22 SEO metadata component for public site

One presentational SeoHead renders the full OG/Twitter/JSON-LD head surface at prerender via typed schema.org builders. Per-medium release schema, config-sourced canonicals, 404 noindex. Zero CMS change.

DeepDrftPublic.Client/Pages/MixDetail.razor

@@ -2,8 +2,7 @@
 @using DeepDrftPublic.Client.Controls
 @using DeepDrftPublic.Client.Services
 @inherits ReleaseDetailBase
-
-<PageTitle>@(ViewModel.Release?.Title ?? "Mix") - DeepDrft</PageTitle>
+@inject SeoOptions Seo
 
 @if (ViewModel.IsLoading)
 {
@@ -32,6 +31,11 @@ else if (ViewModel.NotFound || ViewModel.Release is null)
 else
 {
     var release = ViewModel.Release;
+    var mixTracks = ViewModel.Track is not null ? new[] { ViewModel.Track } : null;
+
+    @* SEO head — fed from the same bridged release + single track, so prerender and WASM render identical
+       tags (AC6). MusicRecording with ISO-8601 duration from the track (§3.4). *@
+    <SeoHead Model="@SeoModel.ForRelease(Seo, release, mixTracks)" />
 
     @* Full-page waveform sits behind the scaffold content. The scaffold's container is positioned
        above it via the mix-detail-foreground stacking context. TrackId lets the visualizer couple to