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>