[BUG] — `meta_data.pathExistedExclude`: cross-namespace path scan + `NOT IN` + unindexed `path`

- severity: Medium; files: `mappers/meta-data-sqlmap.xml:154-163`
- description: Unlike `pathExisted` (which filters `path AND namespace_id`), this query has no `namespace_id` filter, so it scans every `meta_data` row whose `path` matches across all namespaces. `path` is unindexed (PERF-A9) → full table scan; `NOT IN` with a list further inhibits index usage. Also a correctness concern: path-uniqueness is supposed to be per-namespace.
- impact: Each call walks the whole `meta_data` table; correctness drift across namespaces.
- suggested_fix: Add `AND namespace_id = #{namespaceId}` and index `(namespace_id, path)`; rewrite `NOT IN` as `NOT EXISTS`.
- confidence: High

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._

Start with mappers/meta-data-sqlmap.xml:154-163 and compare pathExisted with pathExistedExclude, then inspect how meta_data indexes are defined. Verify the query’s namespace and exclusion behavior across multiple namespaces and confirm the index and NOT EXISTS changes with the relevant database tests or checks; done means scoped, efficient path scanning with unchanged intended results.

Written by the indexing model from the issue text.