Raise the SQLite busy timeout to 30s for serialized migrators

Why: with _txlock=immediate the whole migration pass holds the write lock; four racing migrators on a slow CI disk exceeded the 5s busy timeout (job 2285: SQLITE_BUSY in TestMigrateRacingSQLite) although the local tier passes.

Evidence: go test -count=3 on the migration atomicity suite green locally; CI failure log cited in the Dev PR.

Impact: local SQLite tier only - lock contention waits up to 30s instead of erroring; no schema or API change.

Refs: Dev PR #325; plan KTD8.

internal/store/store.go

@@ -86,7 +86,9 @@ func OpenSQLite(path string) (*Store, error) {
 	}
 	// _txlock=immediate：写事务在 BEGIN 即取库写锁（而非首个写语句升级），
 	// 跨进程迁移/写路径依赖 busy_timeout 排队而不是中途 SQLITE_BUSY（KTD8）。
-	dsn := fmt.Sprintf("file:%s?_pragma=busy_timeout(5000)&_pragma=journal_mode(WAL)&_pragma=foreign_keys(1)&_txlock=immediate", path)
+	// busy_timeout 30s：迁移竞争测试在慢速 CI 盘上 4 个迁移者串行全程可超 5s
+	// （实测 job 2285 SQLITE_BUSY）；本地档宁可排队不报错。
+	dsn := fmt.Sprintf("file:%s?_pragma=busy_timeout(30000)&_pragma=journal_mode(WAL)&_pragma=foreign_keys(1)&_txlock=immediate", path)
 	db, err := sql.Open("sqlite", dsn)
 	if err != nil {
 		return nil, err