feat: Phase 1 Group 4 — add account_id to user_folders and user_pinned_trees

Backfill: user_id → users.account_id

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/folder.py

@@ -46,6 +46,12 @@ class UserFolder(Base):
         nullable=False,
         index=True
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     name: Mapped[str] = mapped_column(String(100), nullable=False)
     color: Mapped[str] = mapped_column(String(7), nullable=False, default="#6366f1")
     icon: Mapped[str] = mapped_column(String(50), nullable=False, default="folder")