fix(contact): auto-invalidate in-memory caches when contact.db is re-decrypted (#98)

## Problem

`_contact_names`, `_contact_full`, `_contact_tags`, and `_self_username`
are populated lazily on first access and never invalidated for the
process lifetime. When `contact.db` is re-decrypted (new contact added,
remark or group name edited, etc.) the on-disk DB updates but the
running MCP server keeps serving stale data — newly-added contacts are
invisible to `resolve_username` and downstream tools until the server
is restarted.

## Fix

Track the mtime of the contact.db backing file. On every
`_get_contact_db_path()` call (which all contact accessors go through),
compare against `_contact_db_mtime`; if it changed, clear all four
caches and record the new mtime. Lookups that don't trigger a real
re-decryption pay only one `os.path.getmtime()` syscall.

The function is reorganized so `_get_contact_db_path()` is the single
source of truth for both "where is contact.db" and "do we need to
invalidate" — `get_contact_names` and `_load_contact_tags` consult it
unconditionally before the early-return on the populated cache.

Also reorders `_get_self_username` to call `get_contact_names()` first
(which now triggers the mtime check via the path lookup) before
returning a cached `_self_username` — otherwise the rename case would
still resolve to the stale name.

## Tests

Baseline 183 → 183 passing, 0 regressions.

The pattern (mtime-track + invalidate-on-change) mirrors the existing
behaviour of DBCache, which already re-decrypts contact.db when the
source mtime changes; this fix closes the symmetric gap on the
in-memory side.

## Scope

- `mcp_server.py` only.
- No public surface change. Affects the contact-cache layer's behaviour
  on re-decryption — previously: stale until restart; now: refreshed
  on next contact-related call.

mcp_server.py

@@ -225,6 +225,15 @@ _contact_names = None  # {username: display_name}
 _contact_full = None   # [{username, nick_name, remark}]
 _contact_tags = None   # {label_id: {name, sort_order, members: [{username, display_name}]}}
 _self_username = None
+_contact_db_mtime = 0  # mtime of the decrypted contact.db when caches were last populated
+
+
+def _invalidate_contact_caches():
+    global _contact_names, _contact_full, _contact_tags, _self_username
+    _contact_names = None
+    _contact_full = None
+    _contact_tags = None
+    _self_username = None
 _XML_UNSAFE_RE = re.compile(r'<!DOCTYPE|<!ENTITY', re.IGNORECASE)
 _XML_PARSE_MAX_LEN = 20000
 _QUERY_LIMIT_MAX = 500
@@ -246,47 +255,57 @@ def _load_contacts_from(db_path):
     return names, full
 
 
+def _get_contact_db_path():
+    """获取 contact.db 路径并按 mtime 决定是否清缓存。
+
+    优先实时解密路径（DBCache 已经按源 mtime 触发重解密），其次回退到
+    静态已解密副本。任何一次 mtime 变化都使内存缓存失效，避免新增联系人
+    或改名/改备注后 MCP 查询仍读到旧数据。
+    """
+    global _contact_db_mtime
+
+    path = _cache.get(os.path.join("contact", "contact.db"))
+    if not path:
+        pre = os.path.join(DECRYPTED_DIR, "contact", "contact.db")
+        path = pre if os.path.exists(pre) else None
+
+    if not path:
+        return None
+
+    try:
+        mt = os.path.getmtime(path)
+    except OSError:
+        return path
+
+    if mt != _contact_db_mtime:
+        _invalidate_contact_caches()
+        _contact_db_mtime = mt
+
+    return path
+
+
 def get_contact_names():
     global _contact_names, _contact_full
+
+    path = _get_contact_db_path()
+    if not path:
+        return {}
+
     if _contact_names is not None:
         return _contact_names
 
-    # 优先用已解密的 contact.db
-    pre_decrypted = os.path.join(DECRYPTED_DIR, "contact", "contact.db")
-    if os.path.exists(pre_decrypted):
-        try:
-            _contact_names, _contact_full = _load_contacts_from(pre_decrypted)
-            return _contact_names
-        except Exception:
-            pass
-
-    # 实时解密
-    path = _cache.get(os.path.join("contact", "contact.db"))
-    if path:
     try:
         _contact_names, _contact_full = _load_contacts_from(path)
         return _contact_names
     except Exception:
-            pass
-
         return {}
 
 
 def get_contact_full():
-    global _contact_full
-    if _contact_full is None:
     get_contact_names()
     return _contact_full or []
 
 
-def _get_contact_db_path():
-    """获取 contact.db 路径（优先已解密，其次实时解密）"""
-    pre = os.path.join(DECRYPTED_DIR, "contact", "contact.db")
-    if os.path.exists(pre):
-        return pre
-    return _cache.get(os.path.join("contact", "contact.db"))
-
-
 def _extract_pb_field_30(data):
     """从 extra_buffer (protobuf) 中提取 Field #30 的字符串值（联系人标签ID）"""
     if not data:
@@ -335,13 +354,14 @@ def _extract_pb_field_30(data):
 def _load_contact_tags():
     """加载并缓存联系人标签数据"""
     global _contact_tags
-    if _contact_tags is not None:
-        return _contact_tags
 
     db_path = _get_contact_db_path()
     if not db_path:
         return {}
 
+    if _contact_tags is not None:
+        return _contact_tags
+
     try:
         conn = sqlite3.connect(db_path)
     except Exception:
@@ -483,13 +503,15 @@ def _collapse_text(text):
 
 def _get_self_username():
     global _self_username
-    if _self_username:
-        return _self_username
 
     if not DB_DIR:
         return ''
 
     names = get_contact_names()
+
+    if _self_username:
+        return _self_username
+
     account_dir = os.path.basename(os.path.dirname(DB_DIR))
     candidates = [account_dir]