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/endpoints/shared.py

@@ -0,0 +1,70 @@
+"""Public endpoints for accessing shared content (no authentication required)."""
+from datetime import datetime, timezone
+from typing import Annotated
+from fastapi import APIRouter, Depends, HTTPException, status
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select
+from sqlalchemy.orm import selectinload
+
+from app.core.database import get_db
+from app.models.tree import Tree
+from app.models.tree_share import TreeShare
+from app.schemas.tree import SharedTreeResponse
+
+router = APIRouter(prefix="/shared", tags=["shared"])
+
+
+@router.get("/{share_token}", response_model=SharedTreeResponse)
+async def get_shared_tree(
+    share_token: str,
+    db: Annotated[AsyncSession, Depends(get_db)]
+):
+    """Get a tree by its share token (PUBLIC endpoint - no auth required).
+
+    Returns 404 if:
+    - Share token doesn't exist
+    - Share token has expired
+    - Tree is not active
+    """
+    # Look up share token
+    result = await db.execute(
+        select(TreeShare)
+        .options(selectinload(TreeShare.tree).selectinload(Tree.tags))
+        .where(TreeShare.share_token == share_token)
+    )
+    tree_share = result.scalar_one_or_none()
+
+    if not tree_share:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Share link not found or has been revoked"
+        )
+
+    # Check expiration
+    if tree_share.expires_at and tree_share.expires_at < datetime.now(timezone.utc):
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Share link has expired"
+        )
+
+    # Check tree is active
+    tree = tree_share.tree
+    if not tree or not tree.is_active:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Tree not found"
+        )
+
+    # Build response (minimal info for public access)
+    return SharedTreeResponse(
+        id=tree.id,
+        name=tree.name,
+        description=tree.description,
+        category=tree.category,
+        tree_structure=tree.tree_structure,
+        tags=tree.tag_names,
+        version=tree.version,
+        allow_forking=tree_share.allow_forking,
+        created_at=tree.created_at,
+        updated_at=tree.updated_at
+    )