[Epic] Fix SSE re-emit race after async UPDATEs

[jakebromberg]

Problem

Multiple async post-INSERT UPDATEs on flowsheet rows fire after the synchronous liveFs SSE refetch event. Connected consumers fetch the row before the UPDATE lands, see NULL, and never see a second event. This affects iOS users (tubafrenzy mode, ~100% today) and dj-site once it migrates to SSE.

The fix shape is shared across both children: a resolved-callback mechanism on the fire-and-forget helper that fires after the UPDATE commits with at least one user-visible field non-null, routed through a single in-process notify point that drives the playlist-proxy re-enrich and a richer liveFs broadcast (type='metadata-updated' / 'linkage-updated', with the changed fields).

Children

• Re-emit liveFs SSE event after metadata enrichment UPDATE lands #628 — re-emit liveFs after metadata enrichment UPDATE lands (fireAndForgetMetadataForRow). The load-bearing fix: introduces the resolved-callback mechanism and the playlist-proxy re-enrich notify point.
• Re-emit on LML linkage UPDATE (album_id race; sibling of #628) #644 — re-emit on LML linkage UPDATE (fireAndForgetLinkage). Same race shape, different field set. Mirrors the mechanism Re-emit liveFs SSE event after metadata enrichment UPDATE lands #628 introduces; depends on Re-emit liveFs SSE event after metadata enrichment UPDATE lands #628 landing first.

Both children share the same notify-point machinery; this parent serves as a single tracking entry for the work.

Adjacent

• PR Denormalize library.artwork_url onto flowsheet on bin-pick #643 (open) — bin-pick artwork denorm. Eliminates the metadata race for the body.album_id !== undefined branch entirely (no LML lookup needed when the album row is in hand). Strictly complementary to Re-emit liveFs SSE event after metadata enrichment UPDATE lands #628 — it shrinks the surface that Re-emit liveFs SSE event after metadata enrichment UPDATE lands #628's callback has to cover.

Acceptance

• Re-emit liveFs SSE event after metadata enrichment UPDATE lands #628 closed (resolved-callback mechanism in place; playlist-proxy receives in-process notify; tubafrenzy-mode iOS sees post-enrichment artwork within ~3s).
• Re-emit on LML linkage UPDATE (album_id race; sibling of #628) #644 closed (linkage UPDATE callback wired through the same notify point; rotation_bin / on_streaming visibility updates within ~1s).

Out of scope

• dj-site free-form-entry latency (tracked at Reduce metadata visibility latency on free-form flowsheet entries (60s polling) dj-site#476).
• iOS BS-mode liveFs subscription (tracked at Subscribe to Backend-Service liveFs SSE for metadata updates (BS-mode) wxyc-ios-64#227, gated on BS-mode rollout).