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/alembic/versions/022_add_tree_forking.py

@@ -0,0 +1,73 @@
+"""add tree forking support with lineage tracking
+
+Revision ID: 022
+Revises: 021
+Create Date: 2026-02-07
+
+Adds fork relationship and lineage tracking to trees table:
+- parent_tree_id: Points to immediate parent tree (NULL for root trees)
+- fork_reason: Optional engineer note explaining fork purpose
+- parent_updated_at: Snapshot of parent's updated_at at fork time
+- root_tree_id: Points to original tree at root of fork chain
+- fork_depth: How many forks deep (1 = direct fork, 2 = fork of fork, etc.)
+
+Fork on parent delete behavior: SET NULL (orphaned forks survive)
+Root tree delete behavior: SET NULL (lineage preserved via fork_depth)
+"""
+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 = '022'
+down_revision: Union[str, None] = '021'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add fork tracking columns
+    op.add_column('trees', sa.Column('parent_tree_id', UUID(as_uuid=True), nullable=True))
+    op.add_column('trees', sa.Column('fork_reason', sa.String(255), nullable=True))
+    op.add_column('trees', sa.Column('parent_updated_at', sa.DateTime(timezone=True), nullable=True))
+
+    # Add fork lineage columns
+    op.add_column('trees', sa.Column('root_tree_id', UUID(as_uuid=True), nullable=True))
+    op.add_column('trees', sa.Column('fork_depth', sa.Integer, nullable=False, server_default='0'))
+
+    # Add foreign key constraints
+    op.create_foreign_key(
+        'fk_trees_parent_tree_id',
+        'trees', 'trees',
+        ['parent_tree_id'], ['id'],
+        ondelete='SET NULL'
+    )
+    op.create_foreign_key(
+        'fk_trees_root_tree_id',
+        'trees', 'trees',
+        ['root_tree_id'], ['id'],
+        ondelete='SET NULL'
+    )
+
+    # Add indexes for fork queries
+    op.create_index('ix_trees_parent_tree_id', 'trees', ['parent_tree_id'])
+    op.create_index('ix_trees_root_tree_id', 'trees', ['root_tree_id'])
+
+    # Composite index for fork analytics (descendants + depth sorting)
+    op.create_index('ix_trees_fork_analytics', 'trees', ['root_tree_id', 'fork_depth'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_trees_fork_analytics', table_name='trees')
+    op.drop_index('ix_trees_root_tree_id', table_name='trees')
+    op.drop_index('ix_trees_parent_tree_id', table_name='trees')
+    op.drop_constraint('fk_trees_root_tree_id', 'trees', type_='foreignkey')
+    op.drop_constraint('fk_trees_parent_tree_id', 'trees', type_='foreignkey')
+    op.drop_column('trees', 'fork_depth')
+    op.drop_column('trees', 'root_tree_id')
+    op.drop_column('trees', 'parent_updated_at')
+    op.drop_column('trees', 'fork_reason')
+    op.drop_column('trees', 'parent_tree_id')