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).

chat_export_helpers.py

@@ -113,26 +113,78 @@ def _format_video_message(content):
     return f"[视频] {playlength}秒" if playlength else "[视频]"
 
 
+def _extract_transfer_extras(content):
+    """Detect appmsg type=2000 and return structured transfer fields, else None.
+
+    Reuses mcp_server._extract_transfer_info so the schema/version-quirks logic
+    lives in one place. Empty values are dropped to keep the export compact.
+    Numeric timestamps are returned as ints (consistent with the top-level
+    `timestamp` field), not iso strings — downstream consumers can format.
+    """
+    if not content or '<appmsg' not in content:
+        return None
+    root = mcp_server._parse_app_message_outer(content)
+    if root is None:
+        return None
+    appmsg = root.find('.//appmsg')
+    if appmsg is None:
+        return None
+    app_type = mcp_server._parse_int(
+        mcp_server._collapse_text(appmsg.findtext('type') or ''), 0
+    )
+    if app_type != 2000:
+        return None
+
+    info = mcp_server._extract_transfer_info(appmsg)
+    if not info:
+        return None
+
+    out = {}
+    if info['paysubtype_label']:
+        out['direction'] = info['paysubtype_label']
+    for k in ('paysubtype', 'fee_desc', 'pay_memo',
+              'payer_username', 'receiver_username',
+              'transfer_id', 'transcation_id', 'pay_msg_id'):
+        v = info.get(k)
+        if v:
+            out[k] = v
+    for k in ('begin_transfer_time', 'invalid_time'):
+        v = mcp_server._parse_int(info.get(k) or '', 0)
+        if v:
+            out[k] = v
+    return out or None
+
+
 def _extract_content(local_id, local_type, content, ct, chat_username, chat_display_name):
+    """Return (rendered_text, extras_dict). Either may be None.
+
+    extras carries structured fields for non-text message types where caller
+    wants more than the human-readable string (currently: transfer). Future
+    additions (video号 metadata, merged-forward expansion, …) can flow through
+    the same channel without changing the caller signature.
+    """
     content = mcp_server._decompress_content(content, ct)
     if content is None:
-        return None
+        return None, None
 
     base, _ = mcp_server._split_msg_type(local_type)
     if base == 1:
-        return content or ""
+        return (content or ""), None
     if base == 43:
-        return _format_video_message(content)
+        return _format_video_message(content), None
     if base == 47:
-        return _format_sticker_message(content)
+        return _format_sticker_message(content), None
     if base == 49:
-        return mcp_server._format_app_message_text(
+        rendered = mcp_server._format_app_message_text(
             content, local_type, False, chat_username, chat_display_name, {}
         )
+        transfer = _extract_transfer_extras(content)
+        extras = {'type': 'transfer', 'transfer': transfer} if transfer else None
+        return rendered, extras
     if base == 50:
-        return mcp_server._format_voip_message_text(content)
+        return mcp_server._format_voip_message_text(content), None
     if base == 10000:
-        return _format_system_message(content)
+        return _format_system_message(content), None
     if base == 10002:
-        return "[撤回消息]"
-    return None
+        return "[撤回消息]", None
+    return None, None