Add public/private visibility for trees

- Add is_public field to Tree model (private by default)
- Update access control: users see default trees, public trees, or their own
- Update all tree endpoints (list, search, get, categories) with new visibility logic
- Default/system trees are automatically marked as public
- Add migration 004 to add is_public column and update existing defaults
- Fix pydantic settings to ignore extra env vars (DATABASE_URL_SYNC)

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

backend/app/schemas/tree.py

@@ -12,6 +12,7 @@ class TreeBase(BaseModel):
 
 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)")
 
 
@@ -20,6 +21,7 @@ class TreeUpdate(BaseModel):
     description: Optional[str] = None
     category: Optional[str] = Field(None, max_length=100)
     tree_structure: Optional[dict[str, Any]] = None
+    is_public: Optional[bool] = None
     is_active: Optional[bool] = None
 
 
@@ -29,6 +31,7 @@ class TreeResponse(TreeBase):
     author_id: Optional[UUID] = None
     team_id: Optional[UUID] = None
     is_active: bool
+    is_public: bool
     is_default: bool
     version: int
     created_at: datetime
@@ -45,6 +48,7 @@ class TreeListResponse(BaseModel):
     description: Optional[str] = None
     category: Optional[str] = None
     is_active: bool
+    is_public: bool
     is_default: bool
     version: int
     usage_count: int