feat(write_attachments): add --append mode, fix batch timing and SQL Server param limit

- Add --append flag: diff source-table 总排号 against existing
  Common.Attachment SN and classify/write only the missing ones. --limit
  caps the per-run append count, --order sets the direction. Backed by
  new fetch_existing_attachment_sns() in db.py.
- Fix batch elapsed-time accounting: summarize_results() summed each
  task's per-item elapsed_ms, which overcounts under ThreadPoolExecutor
  concurrency (cumulative work time, not real wall-clock — 100 tasks on
  8 workers reported ~5x the actual runtime). Callers now time
  classify_batch() via perf_counter and pass wall_clock_ms; both the
  write_attachments [汇总] line and main.py --summary report wall-clock
  separately from the cumulative sum.
- Format durations >=1s in seconds (88851.4 ms -> 88.85 s) in the
  human-readable [汇总] line; structured JSON --summary fields stay in ms.
- Chunk all IN (...) lists to 2000 items to respect SQL Server's 2100
  bind-parameter hard limit (previously --append --limit 5000 failed at
  the fetch step with "COUNT 字段不正确"). Applied to fetch_params_by_ids
  (sn / id paths) and to the DELETE inside upsert_attachments.

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

main.py

@@ -62,6 +62,7 @@ import argparse
 import json
 import logging
 import sys
+import time
 from pathlib import Path
 
 sys.path.insert(0, str(Path(__file__).resolve().parent / "src"))
@@ -166,6 +167,10 @@ def main() -> None:
     # 只能从"关"打开成"开"，不支持反过来用命令行强制关闭配置文件里已打开的设置。
     enable_other = bool(args.enable_other) or bool(cfg["business"].get("enable_other_category", False))
 
+    # 整批分类的真实墙钟：在并发 classify_batch 外层计时（perf_counter）。
+    # summarize_results 累加的 total_elapsed_ms 是各任务自身耗时的总和（并发下的
+    # 累计工作量，会明显大于墙钟），二者之比 ≈ 并发增益；wall_clock_ms 才是真实墙钟。
+    t0 = time.perf_counter()
     results = classify_batch(
         db_cfg=cfg["database"],
         llm_cfg=cfg["llm"],
@@ -175,6 +180,7 @@ def main() -> None:
         log_dir=log_dir,
         enable_other=enable_other,
     )
+    wall_ms = (time.perf_counter() - t0) * 1000
 
     if args.pretty:
         print(json.dumps(results, ensure_ascii=False, indent=2))
@@ -183,7 +189,7 @@ def main() -> None:
             print(json.dumps(r, ensure_ascii=False))
 
     if args.summary:
-        summary = summarize_results(results)
+        summary = summarize_results(results, wall_clock_ms=wall_ms)
         print("--- 运行汇总 ---", file=sys.stderr)
         print(json.dumps(summary, ensure_ascii=False, indent=2), file=sys.stderr)