Thread cluster scope through external BLAST facade outbound calls

Motivation

Issue #26 tracks outbound OpenAPI calls that still resolve against global runtime keys when the call site has no cluster context. The core resolver already supports per-cluster context, but several facade and fallback routes still called it without threading scope.

User-facing change

Routes that can be cluster-scoped now accept optional subscription_id / resource_group / cluster_name and forward those values through outbound OpenAPI calls.

• POST /api/v1/elastic-blast/submit forwards scope to external_blast.ready and external_blast.submit_job.
• GET /api/v1/elastic-blast/jobs/{job_id}, /events, /manifest, and /files/{file_id} forward scope to external_blast.get_job / stream_file.
• GET /api/v1/elastic-blast/jobs accepts optional scope and forwards it to the cached external list fetch.
• Fallback paths in /api/blast/jobs/{job_id} and /api/blast/jobs/{job_id}/results (including result file streaming) now pass provided scope when calling external OpenAPI.

All added parameters default to empty values, so existing callers keep the legacy behavior unless they provide scope explicitly.

API / IaC diff summary

• Updated route handlers:
• api/routes/elastic_blast.py
• api/routes/blast/jobs.py
• api/routes/blast/results.py
• Updated external client entry points to thread scope:
• api/services/external_blast.py
• Added route-level regression tests in api/tests/test_external_blast_api.py.
• No IaC change. No new dependency.

Validation evidence

• .venv/bin/ruff check api/services/external_blast.py api/routes/elastic_blast.py api/routes/blast/jobs.py api/routes/blast/results.py api/tests/test_external_blast_api.py — clean.
• .venv/bin/pytest -q api/tests/test_external_blast_api.py api/tests/test_external_blast_cluster_resolver.py — 108 passed.
• .venv/bin/pytest -q -n 0 api/tests — 3832 passed, 3 skipped, 79 deselected.