feat: upgrade tree deletion to soft delete with deleted_at timestamp

Adds deleted_at and deleted_by columns to trees table for proper soft
delete tracking. Supports future 30-day restore window functionality.
The delete endpoint now sets both is_active=False (backward compat) and
deleted_at/deleted_by. Migration backfills existing is_active=False rows.

Fixed ambiguous FK relationship between User/Tree models by adding
explicit foreign_keys to both sides of the author relationship.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/alembic/versions/015_add_deleted_at_to_trees.py

@@ -0,0 +1,38 @@
+"""add deleted_at and deleted_by columns to trees
+
+Revision ID: 015
+Revises: 014
+Create Date: 2026-02-05
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID
+
+
+# revision identifiers, used by Alembic.
+revision: str = '015'
+down_revision: Union[str, None] = '014'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('trees', sa.Column('deleted_at', sa.DateTime(timezone=True), nullable=True))
+    op.add_column('trees', sa.Column('deleted_by', UUID(as_uuid=True), nullable=True))
+    op.create_index('ix_trees_deleted_at', 'trees', ['deleted_at'])
+    op.create_foreign_key('fk_trees_deleted_by_users', 'trees', 'users', ['deleted_by'], ['id'])
+
+    # Backfill: convert existing is_active=False to deleted_at=now()
+    op.execute("UPDATE trees SET deleted_at = NOW() WHERE is_active = FALSE")
+
+
+def downgrade() -> None:
+    # Convert back
+    op.execute("UPDATE trees SET is_active = FALSE WHERE deleted_at IS NOT NULL")
+    op.drop_constraint('fk_trees_deleted_by_users', 'trees', type_='foreignkey')
+    op.drop_index('ix_trees_deleted_at', table_name='trees')
+    op.drop_column('trees', 'deleted_by')
+    op.drop_column('trees', 'deleted_at')

backend/app/api/endpoints/trees.py

@@ -1,3 +1,4 @@
+from datetime import datetime, timezone
 from typing import Annotated, Optional
 from uuid import UUID
 from fastapi import APIRouter, Depends, HTTPException, status, Query
@@ -508,7 +509,7 @@ async def delete_tree(
     db: Annotated[AsyncSession, Depends(get_db)],
     current_user: Annotated[User, Depends(require_admin)]
 ):
-    """Soft delete a tree (admin only). Sets is_active to False."""
+    """Soft delete a tree (admin only). Sets deleted_at timestamp and is_active=False."""
     result = await db.execute(select(Tree).where(Tree.id == tree_id))
     tree = result.scalar_one_or_none()
 
@@ -519,6 +520,8 @@ async def delete_tree(
         )
 
     tree.is_active = False
+    tree.deleted_at = datetime.now(timezone.utc)
+    tree.deleted_by = current_user.id
     await log_audit(db, current_user.id, "tree.delete", "tree", tree.id,
                     {"tree_name": tree.name})
     await db.commit()

backend/app/models/tree.py

@@ -50,6 +50,16 @@ class Tree(Base):
     is_active: Mapped[bool] = mapped_column(Boolean, default=True)
     is_public: Mapped[bool] = mapped_column(Boolean, default=False, index=True)
     is_default: Mapped[bool] = mapped_column(Boolean, default=False, index=True)
+    deleted_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+        index=True
+    )
+    deleted_by: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id"),
+        nullable=True
+    )
     version: Mapped[int] = mapped_column(Integer, default=1)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
@@ -63,7 +73,7 @@ class Tree(Base):
     usage_count: Mapped[int] = mapped_column(Integer, default=0)
 
     # Relationships
-    author: Mapped[Optional["User"]] = relationship("User", back_populates="trees")
+    author: Mapped[Optional["User"]] = relationship("User", foreign_keys=[author_id], back_populates="trees")
     team: Mapped[Optional["Team"]] = relationship("Team", back_populates="trees")
     sessions: Mapped[list["Session"]] = relationship("Session", back_populates="tree")
 

backend/app/models/user.py

@@ -52,7 +52,7 @@ class User(Base):
 
     # Relationships
     team: Mapped[Optional["Team"]] = relationship("Team", back_populates="users")
-    trees: Mapped[list["Tree"]] = relationship("Tree", back_populates="author")
+    trees: Mapped[list["Tree"]] = relationship("Tree", foreign_keys="[Tree.author_id]", back_populates="author")
     sessions: Mapped[list["Session"]] = relationship("Session", back_populates="user")
     folders: Mapped[list["UserFolder"]] = relationship("UserFolder", back_populates="user")