项目概览、参与者管理初步优化（未接入真实数据）

backend/app/models/study.py

@@ -1,7 +1,7 @@
 import uuid
 from datetime import datetime
 
-from sqlalchemy import DateTime, ForeignKey, String, func
+from sqlalchemy import DateTime, ForeignKey, Integer, String, func
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -18,5 +18,9 @@ class Study(Base):
     protocol_no: Mapped[str | None] = mapped_column(String(100), nullable=True)
     phase: Mapped[str | None] = mapped_column(String(50), nullable=True)
     status: Mapped[str] = mapped_column(String(20), nullable=False, default="DRAFT")
+    visit_interval_days: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    visit_total: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    visit_window_start_offset: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    visit_window_end_offset: Mapped[int | None] = mapped_column(Integer, nullable=True)
     created_by: Mapped[uuid.UUID | None] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())