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/app/api/router.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter
-from app.api.endpoints import auth, trees, sessions, invite, categories, tags, folders, step_categories, steps, admin, accounts, webhooks, shares
+from app.api.endpoints import auth, trees, sessions, invite, categories, tags, folders, step_categories, steps, admin, accounts, webhooks, shares, shared
 
 api_router = APIRouter()
 
@@ -16,3 +16,4 @@ api_router.include_router(admin.router)
 api_router.include_router(accounts.router)
 api_router.include_router(webhooks.router)
 api_router.include_router(shares.router)
+api_router.include_router(shared.router)  # Public endpoints (no auth)