feat(schema): add note_version.pin_kind and pin_label

Spec: docs/superpowers/specs/2026-05-13-note-version-pinning-design.md

- pin_kind: NULL=rolling, 'auto'=stability-scan, 'manual'=user-declared.
- pin_label: NULL for rolling; auto-generated for 'auto'; user-supplied
  string for 'manual' (may be NULL).

No backfill — every existing row stays rolling. The daily auto-pin scan
will catch up on the first run after deploy.

alembic/versions/0045_add_note_version_pin_columns.py

@@ -0,0 +1,35 @@
+"""add pin_kind and pin_label to note_versions
+
+Revision ID: 0045
+Revises: 0044
+Create Date: 2026-05-13
+
+Two additive columns on note_versions to support tiered retention:
+
+- pin_kind: NULL (rolling autosave), 'auto' (system-declared via stability
+  scan), 'manual' (user-declared with optional commit-note label).
+- pin_label: NULL for rolling. Auto-generated for 'auto'; user-supplied
+  for 'manual' (may be NULL).
+
+No backfill — every existing row stays rolling. The auto-pin scan
+(services/version_pinning_scheduler.py, daily 03:00 UTC) will catch up on
+the first scheduled run after deploy.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "0045"
+down_revision = "0044"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("note_versions", sa.Column("pin_kind", sa.Text(), nullable=True))
+    op.add_column("note_versions", sa.Column("pin_label", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("note_versions", "pin_label")
+    op.drop_column("note_versions", "pin_kind")