feat(keys): 自动保存密钥并支持前端回填

- 新增 output/account_keys.json 账号密钥存储（db_key / image_xor_key / image_aes_key）
- 新增 /api/keys 查询已保存密钥；缺失时兜底从账号目录 _media_keys.json 读取图片密钥

- 数据库解密成功后按账号写入 db_key；保存图片密钥时同步写入 store（失败静默不影响主流程）
- 前端解密页进入图片密钥步骤自动回填；进入下一步/跳过时自动保存一次

src/wechat_decrypt_tool/api.py

@@ -10,6 +10,7 @@ from .routers.chat_export import router as _chat_export_router
 from .routers.chat_media import router as _chat_media_router
 from .routers.decrypt import router as _decrypt_router
 from .routers.health import router as _health_router
+from .routers.keys import router as _keys_router
 from .routers.media import router as _media_router
 from .routers.wechat_detection import router as _wechat_detection_router
 
@@ -38,6 +39,7 @@ app.add_middleware(
 app.include_router(_health_router)
 app.include_router(_wechat_detection_router)
 app.include_router(_decrypt_router)
+app.include_router(_keys_router)
 app.include_router(_media_router)
 app.include_router(_chat_router)
 app.include_router(_chat_export_router)