fix(events): dispatch foreign loops through serving loop

[debpalash]

Summary

• route async emitters on foreign loops through the WebSocket serving loop
• add deterministic two-loop regression coverage
• integrate current source fixes from fix(events): sync endpoints dropped their WS events — rename/delete left every tab stale #1562 and perf(watermark): background-prefetch the AudioSeal generator at startup #1577, including fail-open watermark dispatch during bounded shutdown
• correct contributor attribution for First synthesis serializes the AudioSeal watermark load (~42s inline); intermittent 'No module named getopt' skips watermarking #1576

Tests

• HF_HUB_OFFLINE=1 uv run --frozen pytest -q tests/test_event_bus_thread_emit.py
• targeted watermark lifecycle, generation, route coverage, and changelog tests

Stacked on integration branch; do not merge independently to main.

[greptile-apps]

Greptile Summary

The PR routes event broadcasts from foreign asyncio loops onto the WebSocket serving loop and centralizes asynchronous watermark dispatch with fail-open behavior during bounded shutdown.

• Updates generation, batch, and archetype producers to use the shared asynchronous watermark helper.
• Adjusts watermark executor lifecycle admission and adds regression coverage for cross-loop events and shutdown-time watermarking.
• Corrects contributor attribution in the changelog.

Important Files Changed

Filename	Overview
backend/core/event_bus.py	Foreign-loop emitters now schedule broadcasts thread-safely on the loop that owns listener queues and synchronization state.
backend/services/watermark.py	Adds a shared asynchronous watermark dispatcher that preserves successful audio when executor admission is closed during shutdown.
backend/services/model_manager.py	Distinguishes a live executor from one already shutting down when reopening watermark submissions for a lifespan.
backend/api/routers/generation.py	Replaces duplicated executor dispatch at finalization and streaming-preview sites with the shared watermark helper.
backend/api/routers/archetypes.py	Migrates archetype watermarking to the shared helper while retaining its existing bounded dispatch.
backend/api/routers/batch.py	Migrates whole-track batch watermarking to the shared dedicated-pool helper.
tests/test_event_bus_thread_emit.py	Adds deterministic coverage proving a producer on a foreign running loop reaches a listener owned by the serving loop.
tests/test_watermark_prefetch_coldstart.py	Extends bounded-shutdown coverage to verify asynchronous watermark dispatch returns completed audio unchanged while admission is closed.

_{Reviews (1): Last reviewed commit: "Merge remote-tracking branch 'contributo..." | Re-trigger Greptile}