feat(subject): 参与者状态依赖筛选日期并支持后续编辑

- 未填写筛选日期的参与者不再默认进入 SCREENING，状态显示为占位符。
- SubjectUpdate 不再开放 status 入参，改由后端依据日期推导，并新增
  screening_date 字段允许 PM/CRA 后续补录筛选日期。
- 参与者管理页与详情页同步：详情页支持编辑筛选日期，列表页与详情页
  在没有筛选日期时显示占位符而非旧 enum 值。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/test_subject_medication_adherence.py

@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import date, datetime
 from types import SimpleNamespace
 import uuid
 
@@ -12,6 +12,16 @@ def test_subject_update_accepts_actual_medication_count():
     assert payload.actual_medication_count == 12
 
 
+def test_subject_update_accepts_screening_date():
+    payload = SubjectUpdate(screening_date=date(2026, 5, 25))
+
+    assert payload.screening_date == date(2026, 5, 25)
+
+
+def test_subject_update_does_not_expose_status_input():
+    assert "status" not in SubjectUpdate.model_fields
+
+
 def test_subject_read_includes_actual_medication_count():
     subject = SimpleNamespace(
         id=uuid.UUID("00000000-0000-0000-0000-000000000001"),

backend/tests/test_visit_schedule.py

@@ -275,3 +275,9 @@ def test_derive_subject_status_marks_screening_until_enrollment_date_is_filled()
     subject = SimpleNamespace(screening_date=date(2026, 5, 1), enrollment_date=None)
 
     assert derive_subject_status(subject, [], []) == "SCREENING"
+
+
+def test_derive_subject_status_is_empty_before_screening_date_is_filled():
+    subject = SimpleNamespace(screening_date=None, enrollment_date=None)
+
+    assert derive_subject_status(subject, [], []) == ""