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/tree.py

@@ -245,3 +245,37 @@ class TreeValidationResponse(BaseModel):
     """Response for tree validation endpoint."""
     can_publish: bool
     errors: list[ValidationError] = []
+
+
+# --- Pinned Flows Schemas ---
+
+class PinnedFlowResponse(BaseModel):
+    """A pinned flow in the sidebar."""
+    id: UUID
+    tree_id: UUID
+    tree_name: str
+    tree_type: str
+    category_emoji: Optional[str] = None
+    category_name: Optional[str] = None
+    pinned_at: datetime
+    display_order: int
+
+    class Config:
+        from_attributes = True
+
+
+class PinnedFlowsListResponse(BaseModel):
+    """List of pinned flows."""
+    items: list[PinnedFlowResponse]
+    count: int
+
+
+class PinnedFlowReorderItem(BaseModel):
+    """Single item in a reorder request."""
+    tree_id: UUID
+    display_order: int
+
+
+class PinnedFlowReorderRequest(BaseModel):
+    """Request to reorder pinned flows."""
+    order: list[PinnedFlowReorderItem]