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/models/__init__.py

@@ -20,7 +20,7 @@ from .session_share import SessionShare, SessionShareView
 from .account_limit_override import AccountLimitOverride
 from .feature_flag import FeatureFlag, PlanFeatureDefault, AccountFeatureOverride
 from .platform_setting import PlatformSetting
-from .workspace import Workspace
+from .user_pinned_tree import UserPinnedTree
 
 __all__ = [
     "User",
@@ -52,5 +52,5 @@ __all__ = [
     "PlanFeatureDefault",
     "AccountFeatureOverride",
     "PlatformSetting",
-    "Workspace",
+    "UserPinnedTree",
 ]