feat: add tree forking, custom step tracking, and session sharing

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>

backend/app/models/__init__.py

@@ -15,6 +15,7 @@ from .step_category import StepCategory
 from .step_library import StepLibrary, StepRating, StepUsageLog
 from .refresh_token import RefreshToken
 from .audit_log import AuditLog
+from .session_share import SessionShare, SessionShareView
 
 __all__ = [
     "User",
@@ -38,4 +39,6 @@ __all__ = [
     "StepUsageLog",
     "RefreshToken",
     "AuditLog",
+    "SessionShare",
+    "SessionShareView",
 ]