feat(api): append mode for recalled reference images (+ fix recall socket double-emit)

[lstein]

Summary

Adds an append mode to the recall API for reference images, and fixes a latent double-delivery in the recall socket event that the new mode exposed.

feat(api): append mode for recalled reference images. POST /api/v1/recall/{queue_id} previously always replaced the frontend's reference-image list — the recall_parameters_updated handler dispatched refImagesRecalled with a hard-coded replace: true. This adds an append query parameter; when append=true, the recalled ip_adapters / reference_images are added to the existing list (replace: false) instead of replacing it. The flag rides inside the event's parameters dict so the generated client schema needs no regeneration, and it's injected after the persistence loop so it is never stored as a recall parameter. strict and append are mutually exclusive (400). The frontend skips the dispatch entirely when nothing resolved, so a failed append can never clear the user's current reference images.

fix(sockets): emit the recall event once to the owner+admin room union. RecallParametersUpdatedEvent was emitted in two separate socket.io calls — one to user:{user_id}, one to admin. A socket belonging to both rooms (the system user in single-user mode is also an admin, so it joins both) received the event twice. That was invisible for the idempotent scalar/replace recall fields, but the new append mode pushes rather than replaces, so each append showed up as two copies of the same reference image. Fixed by emitting once to the room union [user_room, "admin"] — python-socketio deduplicates recipients across a room list, so a socket in both rooms is delivered to exactly once while genuinely distinct owner/admin sockets each still receive it. Scoped to the recall event only; the other private events are idempotent UI updates where a double delivery is harmless.

QA Instructions

1. Single-user mode, a model that uses reference images (FLUX.2 Klein / FLUX Kontext / Qwen Image Edit). Add one reference image.
2. POST /api/v1/recall/default?append=true with {"reference_images": [{"image_name": "<an output image>"}]}.
3. Expect exactly one new reference image added (the existing one preserved) — not two, and the prior entry not cleared.
4. Repeat with ?append=false (or omitted) → the list is replaced, as before.
5. ?strict=true&append=true → 400.

Automated: tests/app/routers/test_recall_parameters.py (append flag plumbing, strict/append mutual exclusion, not-persisted) and tests/app/routers/test_multiuser_authorization.py::TestWebSocketAuth::test_recall_parameters_emitted_once_to_owner_and_admin_rooms (single emit to the room union). Full recall + multiuser-auth suites pass (153 tests).

Merge Plan

Standard merge. The frontend change is a single dispatch in setEventListeners.tsx (no redux-slice schema change, no migration). Recommend a frontend rebuild on deploy so the append branch takes effect in the served UI.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration — N/A, no slice shape change (uses existing refImagesRecalled with replace: false)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

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