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/schemas/tree.py

@@ -1,5 +1,5 @@
 from datetime import datetime
-from typing import Optional, Any
+from typing import Optional, Any, Literal
 from uuid import UUID
 from pydantic import BaseModel, Field
 
@@ -25,6 +25,7 @@ class TreeCreate(TreeBase):
     tree_structure: dict[str, Any] = Field(..., description="The decision tree structure in JSON format")
     is_public: bool = Field(False, description="Make tree visible to all users")
     is_default: bool = Field(False, description="Mark as a default/system tree (admin only)")
+    status: Literal['draft', 'published'] = Field('published', description="Status: draft or published")
     category_id: Optional[UUID] = Field(None, description="Category ID from tree_categories table")
     tags: Optional[list[str]] = Field(None, max_length=10, description="List of tag names to assign")
 
@@ -37,6 +38,7 @@ class TreeUpdate(BaseModel):
     tree_structure: Optional[dict[str, Any]] = None
     is_public: Optional[bool] = None
     is_active: Optional[bool] = None
+    status: Optional[Literal['draft', 'published']] = None
     tags: Optional[list[str]] = Field(None, max_length=10, description="List of tag names to assign (replaces existing)")
 
 
@@ -70,6 +72,7 @@ class TreeResponse(TreeBase):
     is_active: bool
     is_public: bool
     is_default: bool
+    status: str  # draft or published
     version: int
     created_at: datetime
     updated_at: datetime
@@ -92,6 +95,7 @@ class TreeListResponse(BaseModel):
     is_active: bool
     is_public: bool
     is_default: bool
+    status: str  # draft or published
     version: int
     usage_count: int
     created_at: datetime
@@ -99,3 +103,60 @@ class TreeListResponse(BaseModel):
 
     class Config:
         from_attributes = True
+
+
+# --- Tree Sharing Schemas ---
+
+class TreeShareCreate(BaseModel):
+    """Request to create a share token for a tree."""
+    allow_forking: bool = Field(True, description="Whether recipients can fork this tree")
+    expires_at: Optional[datetime] = Field(None, description="Optional expiration time for the share")
+
+
+class TreeShareResponse(BaseModel):
+    """Response containing share token and URL."""
+    id: UUID
+    tree_id: UUID
+    share_token: str
+    share_url: str
+    allow_forking: bool
+    created_by: UUID
+    created_at: datetime
+    expires_at: Optional[datetime] = None
+
+    class Config:
+        from_attributes = True
+
+
+class TreeVisibilityUpdate(BaseModel):
+    """Request to update tree visibility."""
+    visibility: Literal['private', 'team', 'link', 'public'] = Field(..., description="Visibility level")
+
+
+class SharedTreeResponse(TreeBase):
+    """Public response for shared trees (minimal info)."""
+    id: UUID
+    tree_structure: dict[str, Any]
+    category: Optional[str] = None
+    tags: list[str] = []
+    version: int
+    allow_forking: bool  # From share token
+    created_at: datetime
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+# --- Tree Validation Schemas ---
+
+class ValidationError(BaseModel):
+    """Individual validation error."""
+    field: str
+    message: str
+
+
+class TreeValidationResponse(BaseModel):
+    """Response for tree validation endpoint."""
+    can_publish: bool
+    errors: list[ValidationError] = []