refactor(shared,backend): MigrateToApiBaseUrl nullable, drop null-forgiving operator

null = no migration is the documented contract and the agent already
assigns null when stripping the field from the on-disk cache; annotate
the property as string? so the backend assignment needs no ! suppression.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Backend/AutopilotMonitor.Functions/Functions/Config/GetAgentConfigFunction.cs

@@ -314,7 +314,7 @@ namespace AutopilotMonitor.Functions.Functions.Config
                 DeviceBlocked = killVerdict.IsBlocked,
                 DeviceKillSignal = killVerdict.IsKill,
                 UnblockAt = killVerdict.UnblockAt,
-                MigrateToApiBaseUrl = migrateTarget!,
+                MigrateToApiBaseUrl = migrateTarget,
             });
 
             return response;

src/Shared/AutopilotMonitor.Shared/Models/Config/AgentConfigResponse.cs

@@ -242,7 +242,7 @@ namespace AutopilotMonitor.Shared.Models
         /// config cannot chain another migration), and a kill signal always wins over it.
         /// Old binaries ignore the field. null = no migration.
         /// </summary>
-        public string MigrateToApiBaseUrl { get; set; }
+        public string? MigrateToApiBaseUrl { get; set; }
     }
 
     /// <summary>