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/MediumFields.razor

@@ -29,6 +29,7 @@
         case ReleaseMedium.Session:
             <SessionFields HeroImageFile="HeroImageFile"
                            HeroImageFileChanged="HeroImageFileChanged"
+                           AllowHeroUpload="AllowHeroUpload"
                            Disabled="Disabled" />
             break;
         case ReleaseMedium.Mix:
@@ -49,5 +50,9 @@
     [Parameter] public IBrowserFile? HeroImageFile { get; set; }
     [Parameter] public EventCallback<IBrowserFile?> HeroImageFileChanged { get; set; }
 
+    // Gates the hero file picker in SessionFields. True on the BatchUpload create path;
+    // false/absent on all edit paths so SessionFields falls back to the guidance alert.
+    [Parameter] public bool AllowHeroUpload { get; set; }
+
     [Parameter] public bool Disabled { get; set; }
 }