优化中心联系人展示名称解析

- 新增中心联系人展示名解析服务，支持从逗号分隔用户 ID 解析姓名\n- 中心列表接口返回 contact_display，避免前端依赖项目成员权限才能展示联系人姓名\n- 中心管理和立项会议授权页面优先使用后端联系人展示名\n- 补充联系人解析服务测试和立项会议授权页面展示顺序测试

backend/app/services/site_contact_display.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+import uuid
+from typing import Mapping
+
+
+def parse_contact_user_ids(contact: str | None) -> set[uuid.UUID]:
+    ids: set[uuid.UUID] = set()
+    for token in _contact_tokens(contact):
+        try:
+            ids.add(uuid.UUID(token))
+        except ValueError:
+            continue
+    return ids
+
+
+def build_contact_display(contact: str | None, users: Mapping[uuid.UUID, object]) -> str | None:
+    labels: list[str] = []
+    for token in _contact_tokens(contact):
+        try:
+            user_id = uuid.UUID(token)
+        except ValueError:
+            labels.append(token)
+            continue
+
+        user = users.get(user_id)
+        label = _user_display_name(user)
+        if label:
+            labels.append(label)
+
+    return "、".join(labels) if labels else None
+
+
+def _contact_tokens(contact: str | None) -> list[str]:
+    return [part.strip() for part in str(contact or "").split(",") if part.strip()]
+
+
+def _user_display_name(user: object | None) -> str | None:
+    if user is None:
+        return None
+    for attr in ("full_name", "username", "email"):
+        value = getattr(user, attr, None)
+        if value:
+            return str(value)
+    return None