feat: add visibility filter param and author_name to tree list endpoint

GET /trees now accepts ?visibility=private|team|link|public to scope results.
TreeListResponse includes author_name (full_name or email) and visibility.
Author names fetched in single query to avoid N+1.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/schemas/tree.py

@@ -176,10 +176,12 @@ class TreeListResponse(BaseModel):
     category_info: Optional[CategoryInfo] = None
     tags: list[str] = []  # List of tag names
     author_id: Optional[UUID] = None
+    author_name: Optional[str] = None  # Display name or email of author
     account_id: Optional[UUID] = None
     is_active: bool
     is_public: bool
     is_default: bool
+    visibility: str = 'team'
     status: str  # draft or published
     version: int
     usage_count: int