feat: parse WeChat transfer messages (appmsg type=2000) (#85)

Add structured parsing for transfer messages so they no longer fall
through to the generic `[链接/文件]` fallback in chat history exports.
Mirrors the dispatch + helper pattern PR #65 (merged-forward type=19)
established for `base_type=49` appmsg sub-types.

## What is added

**Helpers (mcp_server.py):**
- `_TRANSFER_PAYSUBTYPE_LABEL` — maps the 6 community-consensus paysubtypes
  (1 发起 / 3 已收款 / 4 已退还 / 5 过期已退还 / 7 待领取 / 8 已领取);
  unknown values degrade to `未知(paysubtype=N)` so a new variant in a
  future WeChat build is visible rather than silently dropped.
- `_extract_transfer_info(appmsg)` — pulls fields out of `<wcpayinfo>`,
  with snake/camelCase fallback (`feedesc`/`feeDesc`, `pay_memo`/`paymemo`)
  observed across WeChat versions.
- `_format_transfer_message_text(appmsg, title)` — one-line render
  for chat history: `[转账·已收款] ¥100.00 备注: lunch`.

**Dispatch (mcp_server.py):**
- `_format_app_message_text` gains an `app_type == 2000` branch that
  routes to `_format_transfer_message_text`. `get_chat_history`,
  `export_chat`, `export_all_chats` and `monitor_web` all inherit
  automatically.

**New MCP tool (mcp_server.py):**
- `decode_transfer(chat_name, local_id, create_time=0)` — full
  structured view: direction, amount, memo, payer/receiver wxid,
  transfer id, transcation id, begin/invalid timestamps. Uses the
  same multi-shard scan + ambiguity-by-create_time pattern as
  `decode_file_message` / `decode_record_item`.

**CLI wrapper:**
- `decode_transfer.py` at the repo root — argparse wrapper that prints
  the same text as the MCP tool, returning non-zero exit when the
  message can't be decoded (script-friendly).

**JSON export (chat_export_helpers.py + export_chat.py + export_all_chats.py):**
- `_extract_content` now returns `(rendered, extras)`. `extras` carries
  structured fields when a message type has more signal than the
  human-readable string (currently: transfers → `type:"transfer" +
  transfer:{direction, fee_desc, pay_memo, ...}`). The channel is
  forward-compatible — future additions (video号 metadata, expanded
  merged-forward, etc.) flow through the same shape without changing
  the caller signature. JSON consumers that only read `content` are
  unaffected; the change is additive.

**monitor_web (monitor_web.py):**
- Backend dispatch branch + orange-yellow `.msg-transfer` card CSS +
  `renderRich` JS handler.

## Tests

12 new cases in `tests/test_record_decoders.py`:

- `TransferPaysubTypeLabelTests` — locks the 6-value label table.
- `ExtractTransferInfoTests` (6 cases) — full field round-trip, missing
  `<wcpayinfo>` fallback, snake/camelCase variants, unknown paysubtype
  degradation, empty paysubtype handling.
- `FormatTransferMessageTextTests` (4 cases) — initiate / received-with-memo /
  missing-wcpayinfo / missing-fee-desc.
- `AppMessageDispatchTransferTests` — `_format_app_message_text` routes
  type=2000 correctly so `get_chat_history` / `export_chat` both pick
  it up.

All fixtures use synthetic placeholder values (`wxid_payer_synth`,
`¥100.00`, `1` + 27×`0`); no real PII or transaction IDs.

## Scope

7 files, +546 / -15 (additions only — no behavior change for existing
message types). All 180 tests pass locally (168 baseline + 12 new).

decode_transfer.py

@@ -0,0 +1,51 @@
+"""
+读取微信转账消息（appmsg type=2000）的结构化字段。
+
+用法:
+    python3 decode_transfer.py <chat_name> <local_id> [<ts>]
+
+参数:
+    <chat_name>  联系人显示名、备注名或 wxid（仅 1v1 聊天有转账消息）。
+    <local_id>   转账消息的 local_id（从 export_chat 输出 / monitor_web 等地方获取）。
+    [<ts>]       可选 unix 时间戳。当 local_id 在多个分片冲突时用它唯一定位。
+
+输出: 多行可读文本，含方向（发起/收款/退还）、金额、备注、付款/收款 wxid、
+      交易号、发起/失效时间。
+
+需先完成 WeChat DB 解密（详见 README）。本 CLI 是 mcp_server.decode_transfer
+工具的命令行包装，输出格式与 MCP 工具一致。
+"""
+import argparse
+import sys
+
+import mcp_server
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(
+        prog="python3 decode_transfer.py",
+        description="读取微信转账消息的结构化字段",
+    )
+    parser.add_argument("chat_name", help="联系人名/备注/wxid")
+    parser.add_argument("local_id", type=int, help="转账消息的 local_id")
+    parser.add_argument(
+        "ts",
+        nargs="?",
+        type=int,
+        default=0,
+        help="消息的 unix 时间戳（跨分片唯一定位时需要，可省略）",
+    )
+    args = parser.parse_args()
+
+    result = mcp_server.decode_transfer(args.chat_name, args.local_id, args.ts)
+    print(result)
+    # 如果工具返回错误文案，退出码非 0 便于 shell 脚本判断
+    if result.startswith(("错误:", "找不到", "不是转账消息", "无法解析", "消息中没有", "消息 content")):
+        return 1
+    if "无法唯一定位" in result:
+        return 2
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())