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/tests/conftest.py

@@ -16,6 +16,8 @@ from app.main import app
 from app.core.database import Base, get_db
 from app.core.config import settings
 
+# Disable invite code requirement for tests
+settings.REQUIRE_INVITE_CODE = False
 
 # Test database URL (separate from production)
 TEST_DATABASE_URL = "postgresql+asyncpg://postgres:postgres@localhost:5432/patherly_test"