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/user_pinned_tree.py

@@ -0,0 +1,37 @@
+import uuid
+from datetime import datetime, timezone
+from sqlalchemy import DateTime, ForeignKey, Integer, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.dialects.postgresql import UUID
+from app.core.database import Base
+
+
+class UserPinnedTree(Base):
+    """Tracks which trees a user has pinned to their sidebar."""
+    __tablename__ = "user_pinned_trees"
+    __table_args__ = (
+        UniqueConstraint('user_id', 'tree_id', name='uq_user_pinned_tree'),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    tree_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("trees.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    display_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    pinned_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc)
+    )