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/tree.py

@@ -15,7 +15,6 @@ if TYPE_CHECKING:
     from app.models.tag import TreeTag
     from app.models.folder import UserFolder
     from app.models.tree_share import TreeShare
-    from app.models.workspace import Workspace
 
 
 class Tree(Base):
@@ -121,14 +120,6 @@ class Tree(Base):
         onupdate=lambda: datetime.now(timezone.utc)
     )
     usage_count: Mapped[int] = mapped_column(Integer, default=0)
-    workspace_id: Mapped[Optional[uuid.UUID]] = mapped_column(
-        UUID(as_uuid=True),
-        ForeignKey("workspaces.id", ondelete="SET NULL"),
-        nullable=True,
-        index=True,
-        comment="Workspace this tree belongs to (organizational context)"
-    )
-
     # Fork tracking
     parent_tree_id: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True),
@@ -192,9 +183,7 @@ class Tree(Base):
         cascade="all, delete-orphan"
     )
 
-    workspace: Mapped[Optional["Workspace"]] = relationship("Workspace", back_populates="trees")
-
-    # New organization relationships
+    # Organization relationships
     category_rel: Mapped[Optional["TreeCategory"]] = relationship("TreeCategory", back_populates="trees")
     tags: Mapped[list["TreeTag"]] = relationship(
         "TreeTag",