feat: remove workspace system, add pinned flows and label renames

Replace workspace system with pinned flows API (pin/unpin/list/reorder).
Rename user-facing labels: Tree→Flow, Procedure→Project. Add sidebar
nav sub-items for flow type filtering. Remove 11 workspace files,
add migrations 037-038, clean all workspace references.

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

backend/app/schemas/category.py

@@ -37,7 +37,6 @@ class CategoryResponse(CategoryBase):
     display_order: int
     is_active: bool
     color: Optional[str] = None
-    workspace_id: Optional[UUID] = None
     created_at: datetime
     updated_at: datetime
     tree_count: int = 0  # Computed field
@@ -55,7 +54,6 @@ class CategoryListResponse(BaseModel):
     display_order: int
     is_active: bool
     color: Optional[str] = None
-    workspace_id: Optional[UUID] = None
     tree_count: int = 0
 
     class Config: