chore: 初始版本提交 - 简化架构 + 轮询改造

- 移除 Motia Streams 实时通信，改用 3 秒轮询
- 简化前端代码，移除冗余组件
- 简化后端架构，准备 FastAPI 重构
- 更新 pixi.toml 环境配置
- 保留 bttoxin_digger_v5_repro 作为参考文档

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

motia-backend/steps/.gitkeep

@@ -0,0 +1 @@
+# Steps directory for Motia API and Python steps

motia-backend/steps/create-task.step.ts

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+/**
+ * Create Task API Step
+ *
+ * Handles POST /tasks with multipart/form-data
+ * - Generates unique sessionId
+ * - Saves uploaded files to staging directory
+ * - Initializes taskState with all steps PENDING
+ * - Checks concurrency and sets QUEUED or PENDING status
+ * - Emits event to trigger pipeline step
+ *
+ * @see Requirements 5.1, 5.2, 1.4, 1.7
+ */
+
+import { defineApiStep } from '@motia/core'
+import { v4 as uuidv4