feat: 新增外部数据源拉取与 DPO 输出格式支持

- 支持从 PostgreSQL 数据库拉取结构化数据作为训练来源
- 新增 DPO (Direct Preference Optimization) 输出类型
- 支持 chosen/rejected 字段的编辑、校验和发布
- 完善数据预处理切分逻辑和元数据管理
- 移除 OCR 扫描 PDF 功能，保持基础文本解析能力

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

backend/tests/test_data_process_migration.py

@@ -37,6 +37,10 @@ def test_runtime_migration_fails_fast_on_incompatible_schema() -> None:
     for value in ("idle", "queued", "running", "completed", "failed", "cancelled"):
         assert f"'{value}'" in sql
     assert "CREATE TABLE IF NOT EXISTS data_process_results" in sql
+    assert "ADD COLUMN IF NOT EXISTS chosen TEXT NOT NULL DEFAULT ''" in sql
+    assert "ADD COLUMN IF NOT EXISTS rejected TEXT NOT NULL DEFAULT ''" in sql
+    assert "ADD COLUMN IF NOT EXISTS original_chosen TEXT" in sql
+    assert "ADD COLUMN IF NOT EXISTS original_rejected TEXT" in sql
     assert sql.count("BEGIN;") == 1
     assert sql.rstrip().endswith("COMMIT;")