feat: implement tree sharing, draft trees, and session-to-tree conversion (Issues #16, #25, #17)

Backend features:
- Tree sharing via secure tokens with expiration (Issue #16)
- Draft tree status with conditional validation (Issue #25)
- Save session as custom tree with fork tracking (Issue #17)
- Tree validation system for publish requirements
- Session-to-tree conversion preserving custom steps

Database migrations:
- 024: Tree sharing (tree_shares table, visibility field)
- 025: Tree status field (draft/published)
- 25b: Merge migration for indexes

New endpoints:
- POST /api/v1/trees/{id}/share - Generate share token
- GET /api/v1/shared/{token} - Public tree access
- POST /api/v1/trees/{id}/can-publish - Validate tree
- POST /api/v1/sessions/{id}/save-as-tree - Convert session

Test coverage:
- 20 tests for draft trees functionality
- 14 tests for session-to-tree conversion
- 15 tests for tree sharing

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/alembic/versions/025_add_tree_status_field.py

@@ -0,0 +1,47 @@
+"""add tree status field for draft/published workflow
+
+Revision ID: 025
+Revises: 25b001abd0f7
+Create Date: 2026-02-08
+
+Adds status field to trees table for draft/published workflow:
+- status: enum ('draft', 'published') - defaults to 'published' for existing trees
+- Drafts allow incomplete/invalid structures for work-in-progress
+- Published trees require validation before saving
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '025'
+down_revision: Union[str, None] = '25b001abd0f7'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add status field with default 'published' (for backward compatibility)
+    op.add_column('trees', sa.Column(
+        'status',
+        sa.String(20),
+        nullable=False,
+        server_default='published',
+        comment="Status: draft (work in progress) or published (validated and available)"
+    ))
+    op.create_index('ix_trees_status', 'trees', ['status'])
+
+    # Add CHECK constraint for status values
+    op.create_check_constraint(
+        'ck_trees_status',
+        'trees',
+        "status IN ('draft', 'published')"
+    )
+
+
+def downgrade() -> None:
+    op.drop_index('ix_trees_status', table_name='trees')
+    op.drop_constraint('ck_trees_status', 'trees', type_='check')
+    op.drop_column('trees', 'status')