fix(data): ETF 身份契约与挂载权限修复（159399 QA 回归）

[问题原因]
前端手动录入表单保存的 market 字段是 SH/SZ/BJ 原文，但 worker 的
split_identity 只接受 market:"cn" 并按代码前缀推交易所，导致
market:"SZ" 的 159399 在发起任何网络请求前就报 unsupported_market。
同时 server 为非 root 容器（uid 65534）准备挂载时，完整文件挂载从不
被 chmod，且所有 chmod 失败都被静默吞掉，worker 容器读数据集可能
Permission denied 且不易定位。

[问题根因]
身份契约在 server 持久化层与 worker 适配层不一致；挂载权限处理是
best-effort 静默吞错，缺少最小作用域约束。

[修复方案]
worker/data.py: market 原样映射 IDENTITY_EXCHANGES（SH/SZ/BJ），
"cn" 保持代码前缀推断（SH=3/6/9 开头，否则 SZ），未知市场仍显式拒绝
绝不猜测；sina 数据源的警告文案改为只陈述可证实事实（无日期参数、
不除权、成交量单位为股且不换算），删除与其它 provider 的 100x 换算
断言。server/src/worker.rs: prepare_mounts 严格化——独立文件挂载仅在
"只读 + /data/ 前缀"范围内 chmod 0644，拒绝符号链接，目录挂载保持
0755/0777，所有 chmod 失败作为错误返回而非吞掉。tests/worker/
test_data.py 新增身份契约用例：SZ/SH 原文、cn 推断不变、SZ 直达
sina provider 的端到端 fetch。

[影响范围]
worker 数据获取链路与 server 任务编排；server/target/release 二进制
已含本改动（2026-09-17 构建）；pytest 数据模块全绿。

1 files changed, 19 insertions, 11 deletions

diff --git a/worker/data.py b/worker/data.py
index 841a1eb..13f8808 100644
--- a/worker/data.py
+++ b/worker/data.py
@@ -66,13 +66,20 @@ def split_identity(instrument: dict) -> tuple[str, str]:
             raise DataError("bad_identity", f"unknown exchange prefix: {exch}")
     else:
         code = symbol
-        market = instrument.get("market", "cn")
-        if market != "cn":
-            raise DataError("unsupported_market", f"market not supported by this adapter: {market}")
-        if len(code) == 6 and code[0] in "369":
-            exch = "SH"
+        market = str(instrument.get("market", "cn")).strip() or "cn"
+        # The server persists the user-facing market field verbatim (SH/SZ/BJ
+        # from the manual-entry form); "cn" (search-sourced items, any case)
+        # keeps the original code-prefix inference. Unknown markets must still
+        # be rejected — never guessed.
+        if market.upper() in IDENTITY_EXCHANGES:
+            exch = market.upper()
+        elif market.lower() == "cn":
+            if len(code) == 6 and code[0] in "369":
+                exch = "SH"
+            else:
+                exch = "SZ"
         else:
-            exch = "SZ"
+            raise DataError("unsupported_market", f"market not supported by this adapter: {market}")
     if not code.isdigit() or len(code) != 6:
         raise DataError("bad_identity", "A-share code must be a 6-digit number")
     return exch, code
@@ -157,11 +164,12 @@ def _fetch_sina(instrument: dict, start: str, end: str,
     # requested window (ISO strings compare lexicographically like dates)
     df = df[(df["date"] >= start) & (df["date"] <= end)].reset_index(drop=True)
     df.attrs["source_warnings"] = [
-        "sina returns the full trading history without date parameters; sliced "
-        "locally to the requested range; data is unadjusted (no adjust parameter)",
-        "sina volume unit is 股 (shares); verified to be 100x the 手 (lots) "
-        "convention used by lot-based providers on the same session "
-        "(2026-09-17 evidence, docs/recovery-01-plan.md)",
+        "sina fund_etf_hist_sina has no date-range parameter: the provider "
+        "returned its available history and it was sliced locally to the "
+        "requested range (actual coverage differences are reported separately)",
+        "sina ETF klines are unadjusted: the interface has no adjust parameter",
+        "sina fund_etf_hist_sina 成交量单位为股；数值按 provider 原样保留，未做换算或取整。"
+        "与其他行情接口对比时，请先核对各 provider 自己公布的口径，不要按惯例直接换算",
     ]
     return FetchResult(df, endpoint, params, raw, "sina")