feat(entitlement): pivot to paid app ownership

Check Steam ownership of the paid app with BIsSubscribedApp; keep Debug fake ownership out of Release. Ref: https://partner.steamgames.com/doc/api/ISteamApps#BIsSubscribedApp

Services/Entitlement/FakeEntitlement.cs

@@ -1,13 +1,11 @@
 namespace ytLive.Services.Entitlement;
 
 /// <summary>
-/// Test double for the entitlement rail. Production never uses it — a fake that could be reached
-/// in a shipped build is a licence bypass, so it lives here only because tests need to drive both
-/// answers without a Steam client or a network.
+/// Development/test ownership result. Release builds exclude this type.
 /// </summary>
 public sealed class FakeEntitlement : IEntitlement
 {
-    public FakeEntitlement(bool isPro = false) => IsPro = isPro;
+    public FakeEntitlement(bool isAppOwned = false) => IsAppOwned = isAppOwned;
 
-    public bool IsPro { get; set; }
+    public bool IsAppOwned { get; set; }
 }