store: add ClaimLegacyOwner one-time upgrade bootstrap

Binds pre-multi-tenancy singleton rows to one named OIDC subject, given at
startup via an env var (wired in Task 11). No-ops once any user exists or
on a genuinely fresh install.

backend/internal/store/legacy_claim.go

@@ -0,0 +1,59 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+)
+
+// ClaimLegacyOwner is a one-time upgrade step, not a normal runtime
+// operation: it binds every pre-multi-tenancy row (user_id IS NULL, left
+// that way by migrations that can't take runtime parameters -- see
+// docs/superpowers/specs/2026-07-25-per-user-profile-design.md) to a single
+// new user identified by oidcSub. Safe to call on every startup: once the
+// users table is non-empty, it's a no-op, so leaving
+// GENIUSRUN_LEGACY_OWNER_OIDC_SUB set after the first successful run causes
+// no harm. Also a no-op on a genuinely fresh install (no legacy profile row
+// to claim at all).
+func (db *DB) ClaimLegacyOwner(ctx context.Context, oidcSub string) error {
+	var userCount int
+	if err := db.QueryRowContext(ctx, `SELECT COUNT(*) FROM users`).Scan(&userCount); err != nil {
+		return fmt.Errorf("count users: %w", err)
+	}
+	if userCount > 0 {
+		return nil // already bootstrapped (either claimed already, or real signups exist)
+	}
+
+	var displayName string
+	err := db.QueryRowContext(ctx, `SELECT name FROM profile WHERE user_id IS NULL LIMIT 1`).Scan(&displayName)
+	if err == sql.ErrNoRows {
+		return nil // fresh install, no pre-existing singleton profile to claim
+	}
+	if err != nil {
+		return fmt.Errorf("find legacy profile: %w", err)
+	}
+
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		return fmt.Errorf("begin claim legacy owner tx: %w", err)
+	}
+	defer tx.Rollback()
+
+	res, err := tx.ExecContext(ctx, `INSERT INTO users (oidc_sub, display_name) VALUES (?, ?)`, oidcSub, displayName)
+	if err != nil {
+		return fmt.Errorf("create legacy owner user: %w", err)
+	}
+	userID, err := res.LastInsertId()
+	if err != nil {
+		return err
+	}
+
+	// table is always one of the fixed literals below, never user input.
+	for _, table := range []string{"profile", "workout_kinds", "activities", "sync_state", "sync_runs"} {
+		if _, err := tx.ExecContext(ctx, `UPDATE `+table+` SET user_id = ? WHERE user_id IS NULL`, userID); err != nil {
+			return fmt.Errorf("claim legacy %s rows: %w", table, err)
+		}
+	}
+
+	return tx.Commit()
+}