Implement three foundational schema features from the design doc:
- Tree forking with lineage tracking (migration 022): parent_tree_id,
root_tree_id, fork_depth columns with self-referential FKs and
composite analytics index
- Custom step enhancement: CustomStepSchema with source tracking
(ad-hoc, step-library, forked-tree) for backward-compatible JSONB
- Session sharing (migration 023): session_shares and session_share_views
tables with account-scoped visibility, cryptographic tokens, view
tracking, and allow_public_shares account policy
Includes 21 new integration tests (9 forking, 12 sharing), SaaS
consultant-recommended denormalizations, rate limiting on public share
access, and test fixture fix for invite code requirement.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
