feat(worker): configurable max parallel task executions

Add a "Max parallel executions" setting to the General settings tab so
the queue can run more than one task concurrently. QueueService now
tracks multiple active slots and reads the limit from app settings each
cycle, so changes take effect without restarting the worker.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Data/Migrations/20260601133737_AddMaxParallelExecutions.cs

@@ -0,0 +1,36 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ClaudeDo.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddMaxParallelExecutions : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<int>(
+                name: "max_parallel_executions",
+                table: "app_settings",
+                type: "INTEGER",
+                nullable: false,
+                defaultValue: 1);
+
+            migrationBuilder.UpdateData(
+                table: "app_settings",
+                keyColumn: "id",
+                keyValue: 1,
+                column: "max_parallel_executions",
+                value: 1);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "max_parallel_executions",
+                table: "app_settings");
+        }
+    }
+}