feat: 支持绑定外部 bt_toxin 数据库 (2025-11-04 更新)

- docker_client.py: run_bttoxin_digger() 新增 bttoxin_db_dir 参数，支持挂载外部数据库
- run_single_fna_pipeline.py: 新增 --bttoxin_db_dir 参数，自动检测 external_dbs/bt_toxin
- README.md: 添加 bttoxin_db 更新说明和 Docker 绑定文档
- external_dbs/bt_toxin: 添加 2025-11-04 版本数据库文件

测试验证: HAN055 样本毒素命名版本号变化 (Cry2Aa9→22, Cry2Ab35→41, Cry1Ia40→42, Vip3Aa7→79)

external_dbs/bt_toxin/db/bt_toxin.pjs

@@ -0,0 +1,22 @@
+{
+  "version": "1.2",
+  "dbname": "bt_toxin",
+  "dbtype": "Protein",
+  "db-version": 5,
+  "description": "bt_toxin20251104.fas",
+  "number-of-letters": 996368,
+  "number-of-sequences": 1199,
+  "last-updated": "2025-11-04T15:35:00",
+  "number-of-volumes": 1,
+  "bytes-total": 1149077,
+  "bytes-to-cache": 1007264,
+  "files": [
+    "bt_toxin.pdb",
+    "bt_toxin.phr",
+    "bt_toxin.pin",
+    "bt_toxin.pot",
+    "bt_toxin.psq",
+    "bt_toxin.ptf",
+    "bt_toxin.pto"
+  ]
+}