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/category.py

@@ -11,7 +11,6 @@ if TYPE_CHECKING:
     from app.models.team import Team
     from app.models.account import Account
     from app.models.user import User
-    from app.models.workspace import Workspace
 
 
 class TreeCategory(Base):
@@ -52,13 +51,6 @@ class TreeCategory(Base):
         String(7), nullable=True, default='#3b82f6',
         comment="Hex color for category dot indicator"
     )
-    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 category belongs to"
-    )
     created_by: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("users.id", ondelete="SET NULL"),
@@ -79,7 +71,6 @@ class TreeCategory(Base):
     account: Mapped[Optional["Account"]] = relationship("Account", foreign_keys=[account_id], back_populates="categories")
     creator: Mapped[Optional["User"]] = relationship("User", foreign_keys=[created_by])
     trees: Mapped[list["Tree"]] = relationship("Tree", back_populates="category_rel")
-    workspace: Mapped[Optional["Workspace"]] = relationship("Workspace", back_populates="categories")
 
     @property
     def is_global(self) -> bool: