WIP checkpoint: Group + membership models + migration

backend/app/models/group.py

@@ -0,0 +1,56 @@
+import enum
+
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, Enum
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+
+from app.database import Base
+
+
+class GroupMemberRole(str, enum.Enum):
+    OWNER = "owner"
+    ADMIN = "admin"
+    MEMBER = "member"
+
+
+class Group(Base):
+    __tablename__ = "groups"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String(255), nullable=False)
+    description = Column(Text, nullable=True)
+    owner_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+    owner = relationship("User", back_populates="owned_groups")
+    members = relationship(
+        "GroupMember",
+        back_populates="group",
+        cascade="all, delete-orphan",
+        order_by="GroupMember.joined_at",
+    )
+
+
+class GroupMember(Base):
+    __tablename__ = "group_members"
+
+    group_id = Column(
+        Integer,
+        ForeignKey("groups.id", ondelete="CASCADE"),
+        primary_key=True,
+    )
+    user_id = Column(
+        Integer,
+        ForeignKey("users.id", ondelete="CASCADE"),
+        primary_key=True,
+    )
+    role = Column(
+        Enum(GroupMemberRole, name="group_member_role", create_type=False),
+        nullable=False,
+        default=GroupMemberRole.MEMBER,
+    )
+    joined_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    group = relationship("Group", back_populates="members")
+    user = relationship("User", back_populates="group_memberships")