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')