UI-SP5b — Executors SSE follow-on (view-free)

[l17728]

Summary

Small follow-on to UI-SP5 (PR #22 8c77ca1). Validates the view-free single-seam SSE architecture against a SECOND consumer.

• Backend (1 additive endpoint, zero migration, zero new dep): GET /api/v1/executors/stream — hand-rolled text/event-stream mirroring tasks_stream.py. Reuses SP3's list_executors_for_principal (tenant filter + system_admin/is_service bypass — same RBAC as the existing GET /executors). 5 s default tick (env DLW_EXECUTORS_STREAM_INTERVAL_SECONDS, clamped [0.5, 60.0]). Same ?max_ticks=N testability hatch (httpx ASGITransport buffering).
• Frontend: useExecutors opts in via the SP5-added streamUrl+applyEvent seam options. Signature/return-shape UNCHANGED.
• View-free proof: pages/Executors.vue NOT modified; SP3's ExecutorsPage.spec.ts + the sp3Composables.spec.ts::useExecutors wires key + status query + interval test both pass UNCHANGED. The second consumer ships without touching ANY view code — exactly as the SP1/SP2/SP3 promise foretold.

Verification

• Backend: 455 pytest (451 prior + 4 new SP5b: unauth/tenant-filter/multi-snapshot/status-filter).
• Frontend: 147 vitest (144 prior + 3 new SP5b: streamUrl+applyEvent wired, streamUrl reactive to filter, applyEvent parses JSON).
• vue-tsc strict 0 errors; eslint --max-warnings=0; vite build success. spectral/swagger/invariant/status-write all green.
• Live headed Playwright smoke: /api/v1/executors/stream SSE request observed in DevTools network on the /executors page; zero real console errors.
• 2-opus pre-execution gate: 0 BLOCKER, 0 IMPORTANT — first "all clear" pre-review across 10 sub-projects, reflecting how well-grooved the architecture is.

Known follow-ups (non-blocking, record-and-accept)

• Status-filter changes mid-stream: the SSE URL is captured once at first snapshot; a later el-select mutation re-renders the page from vue-query's fresh fetch (correct UI), but the old-URL stream keeps pushing until component unmount. Cosmetic; ~1 tick of potentially-stale items before user navigates. Future enhancement: have useLiveResource watch(() => toValue(opts.streamUrl), ...) and reopen on URL change — out of scope here (touches the SP5 seam).
• Vestigial _KEEPALIVE_EVERY_TICKS dead branch (inherited from SP5; harmless).
• OpenAPI text/event-stream block lacks an explicit 401 response (precedent from SP5 — flag for a docs sweep).

Test plan

• CI green
• Squash-merge with --delete-branch

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