Enforce DatabaseSettings validation in AddInfrastructure for all host modes

Move ValidateDataAnnotations/ValidateOnStart for DatabaseSettings from the
API-only OptionsValidationRegistration into AddInfrastructure so that CLI
and MCP hosts also enforce the [Range(1, 300)] constraint on
CommandTimeoutSeconds at startup. Remove the now-duplicate registration from
OptionsValidationRegistration.

Author: Chris0Jeky

backend/src/Taskdeck.Api/Extensions/OptionsValidationRegistration.cs

@@ -45,10 +45,6 @@ public static IServiceCollection AddOptionsValidation(
 
         services.RegisterValidatedOptions<CacheSettings>(configuration, "Cache");
 
-        // ── Settings from Infrastructure (Database resilience) ─────────────
-
-        services.RegisterValidatedOptions<DatabaseSettings>(configuration, "Database");
-
         // ── Cross-property validators (IValidateOptions<T>) ─────────────────
 
         services.AddSingleton<IValidateOptions<WorkerSettings>, WorkerSettingsValidator>();

backend/src/Taskdeck.Infrastructure/DependencyInjection.cs

@@ -23,6 +23,14 @@ public static IServiceCollection AddInfrastructure(this IServiceCollection servi
         var databaseSettings = configuration.GetSection("Database").Get<DatabaseSettings>()
             ?? new DatabaseSettings();
 
+        // Enforce validation for all host modes (API, CLI, MCP).
+        // ValidateOnStart causes an exception at startup if CommandTimeoutSeconds
+        // is out of the [1, 300] range, regardless of which host runs AddInfrastructure.
+        services.AddOptions<DatabaseSettings>()
+            .Bind(configuration.GetSection("Database"))
+            .ValidateDataAnnotations()
+            .ValidateOnStart();
+
         services.AddDbContext<TaskdeckDbContext>(options =>
             options.UseSqlite(connectionString, sqliteOptions =>
             {