fix(cache): invalidate cached results whose output assets are missing

[BerryUIKI]

Closes #111

Summary of Changes

• Validates referenced output assets (\�sset_id\ and /api/v1/assets/{id}/content) upon cache read (\get_async\ / \has_async).
• If an output asset is missing on disk or corrupt, automatically invalidates the stale cache entry from SQLite and memory and returns \None, allowing DAG runners to cleanly recompute outputs.
• Adds
  econcile_orphan_references_async()\ executed at startup to scan SQLite cache entries and prune broken references.
• Adds unit tests in \ est_dag_cache.py\ for cache invalidation on deleted assets and startup reconciliation.