feat(store): add DeleteUser with cascading account deletion

backend/internal/store/users.go

@@ -119,3 +119,17 @@ func (db *DB) ProvisionUser(ctx context.Context, oidcSub, displayName string) (i
 
 	return userID, tx.Commit()
 }
+
+// DeleteUser permanently deletes userID's account. Every row that belongs
+// to it -- profile, workout kinds (and their paces), activities (and their
+// laps/samples/kind_assignments), sync_state, and sync_runs -- cascades
+// away via the ON DELETE CASCADE foreign keys in schema.sql, so this is a
+// single statement rather than per-table deletes. Irreversible; the API
+// layer gates this behind a UI confirmation (see
+// docs/superpowers/specs/2026-07-26-profile-deletion-design.md).
+func (db *DB) DeleteUser(ctx context.Context, userID int64) error {
+	if _, err := db.ExecContext(ctx, `DELETE FROM users WHERE id = ?`, userID); err != nil {
+		return fmt.Errorf("delete user %d: %w", userID, err)
+	}
+	return nil
+}