Add is_default flag for system trees

- Add is_default column to trees table
- Default trees have no author and are visible to all users
- Only admins can create default trees
- Update seed script to mark seeded trees as default
- Update seed script to use CLI auth instead of creating seed user

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/tree.py

@@ -31,6 +31,7 @@ class Tree(Base):
         index=True
     )
     is_active: Mapped[bool] = mapped_column(Boolean, default=True)
+    is_default: Mapped[bool] = mapped_column(Boolean, default=False, index=True)
     version: Mapped[int] = mapped_column(Integer, default=1)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),