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Cache the direct external-BLAST jobs facade

Motivation

Issue #30 tracked the slow /v1/jobs listing surfaced through the dashboard. The investigation listed the direct facade route (GET /api/v1/elastic-blast/jobs → external_blast.list_jobs()) as candidate fix #3: it had no cache, so a stale or unreachable OpenAPI base URL cost the full _LIST_TIMEOUT_SECONDS (5 s) and then a 503 on every poll. The combined /api/blast/jobs route was already cached (_external_list_jobs_cached, 70 s TTL + negative cache + in-flight de-duplication), so the direct facade felt slower by comparison.

User-facing change

  • The direct facade GET /api/v1/elastic-blast/jobs listing is now served through the same shared external-jobs cache the combined route uses. Repeat polls within the TTL are served from memory; a failing sibling is negatively cached so polling does not keep paying the upstream round-trip to learn the same failure.
  • Response shape is unchanged ({"jobs": [...], "count": N}); the 503 openapi_unreachable error contract is unchanged (now short-circuited by the negative cache instead of re-hitting the upstream).

API / IaC diff summary

  • api/routes/elastic_blast.py list_external_blast_jobs now calls external_blast.external_jobs._external_list_jobs_cached({}) and rebuilds the {"jobs": rows, "count": len(rows)} envelope, instead of calling external_blast.list_jobs() synchronously on every request.
  • No IaC change.

Scope notes

  • The biggest #30 cost (per-cluster k8s_get_service_ip 10 s timeout on Stopped/unreachable clusters) is already mitigated by the existing _cluster_power_state_allows_openapi gate in _discover_subscription_clusters (test test_discover_subscription_clusters_skips_stopped).
  • httpx connection reuse for the OpenAPI plane (candidate fix #4) is deferred to the per-cluster resolver work in #26: with the facade now cached, the upstream call rate drops from per-poll to ~once per TTL, so connection reuse is now the lowest-value remaining item and it tangles with the per-cluster base-URL scoping #26 introduces.
  • Sibling-side cold-start / event-loop blocking (candidate fix #5) lives in the elastic-blast-azure repo and is out of scope here.

Validation evidence

  • uv run pytest -q api/tests/test_external_blast_api.py — 79 passed. New tests: test_external_blast_facade_list_is_cached (two polls → one upstream hit), test_external_blast_facade_list_caches_upstream_failure (failing sibling negatively cached → one upstream hit).
  • uv run pytest -q api/tests — 3438 passed, 3 skipped.
  • uv run ruff check api/routes/elastic_blast.py api/tests/test_external_blast_api.py — all checks passed.