fix(cms): gate Session hero input to upload path; warn (not error) on missing hero

Edit forms (BatchEdit/TrackEdit/TrackNew) show the guidance alert instead of an
inert picker, via an AllowHeroUpload flag. Missing-hero nudge is Severity.Warning;
null-ReleaseId hero drop is now logged.

DeepDrftManager/Components/Pages/Tracks/AlbumHeaderFields.razor

@@ -68,6 +68,7 @@
                   @bind-ReleaseType="ReleaseTypeBinding"
                   HeroImageFile="HeroImageFile"
                   HeroImageFileChanged="HeroImageFileChanged"
+                  AllowHeroUpload="AllowHeroUpload"
                   Disabled="Disabled" />
 </MudPaper>
 
@@ -92,6 +93,10 @@
     [Parameter] public IBrowserFile? HeroImageFile { get; set; }
     [Parameter] public EventCallback<IBrowserFile?> HeroImageFileChanged { get; set; }
 
+    // Gates the hero file picker in SessionFields (threaded to MediumFields → SessionFields).
+    // Set true only on the BatchUpload create path; leave false/absent on all edit paths.
+    [Parameter] public bool AllowHeroUpload { get; set; }
+
     // BatchEdit only: when set (and no new file picked), preview the release's current cover.
     // The parent nulls this to drop the preview when the admin clears the existing cover.
     [Parameter] public string? ExistingImagePath { get; set; }