WIP: SSE pub/sub for live escalation arrivals (paused for Codex review)

First half of the WebSocket/SSE push slice. Paused mid-flight to hand
the branch to Codex for outside-voice review before stacking more
commits on top. See .ai/HANDOFF.md for the full pause context + what
to look at.

What's here:
- backend/app/core/escalation_bus.py — module-level singleton in-memory
  pub/sub keyed by account_id. asyncio.Queue per subscriber with
  64-event maxsize and drop-on-full semantics. Designed to be swappable
  for Redis pub/sub when Railway scales past single-replica.
- backend/app/api/endpoints/session_handoffs.py — GET
  /api/v1/ai-sessions/escalations/stream SSE endpoint. Auth via
  require_engineer_or_admin. 25s heartbeat. Account-scoped subscribe
  bound to current_user.account_id.
- backend/app/services/handoff_manager.py — dispatch_escalation_notifications
  now publishes a `handoff_created` event to the bus BEFORE the email
  fan-out, in a try/except so a bus failure can't block email delivery.
- backend/tests/test_escalation_bus.py — 7 unit tests, all green
  standalone (0.14s). Cross-tenant isolation, drop-on-full, no-subscribers.
- backend/tests/test_handoff_manager.py — +1 dispatcher integration test
  (publishes to bus, payload shape).
- backend/tests/test_session_handoffs_api.py — +2 endpoint tests (viewer
  blocked, ready event handshake).

[gstack-context]
Decisions:
  - SSE over WebSocket (one-way, browser EventSource semantics, fewer
    moving parts behind Railway proxy)
  - In-memory bus over Redis for v1 pilot (3 MSPs, single replica)
  - Drop-on-full subscriber queue rather than back-pressure publishers
  - Bus publish ahead of email send, both wrapped in try/except so
    neither can break handoff creation
  - Frontend will be a fetch-based ReadableStream reader matching the
    existing streamDocumentation pattern, not native EventSource
    (custom-header auth)
Remaining (post-Codex):
  - Frontend SSE subscription in EscalationQueue.tsx (slide-in,
    reconnect, tab-title flash, prefers-reduced-motion)
  - Magic-moment handoff-context screen
  - Re-run the full backend test suite to verify the SSE +
    dispatcher integration tests (bus units already green standalone)
Tried:
  - Running the full test suite repeatedly without xdist; the per-test
    DROP SCHEMA + recreate fixture made wall-clock prohibitive when
    multiple stale runs collided on the same Postgres test schema.
    Resolution: -n auto next time.
[/gstack-context]

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/test_handoff_manager.py

@@ -278,6 +278,58 @@ async def test_dispatch_graceful_degradation_when_email_raises(
     assert sent == 0
 
 
+@pytest.mark.asyncio
+async def test_dispatch_publishes_to_escalation_bus(
+    client: AsyncClient, test_user, auth_headers, test_db
+):
+    """dispatch_escalation_notifications puts an event on the in-memory bus
+    so connected SSE subscribers see live arrivals."""
+    from app.core.escalation_bus import bus as escalation_bus
+
+    session = AISession(
+        user_id=test_user["user_data"]["id"],
+        account_id=test_user["user_data"]["account_id"],
+        session_type="guided",
+        intake_type="free_text",
+        intake_content={"text": "x"},
+        problem_summary="VPN down",
+        status="active",
+        confidence_tier="discovery",
+        conversation_messages=[],
+    )
+    test_db.add(session)
+    await test_db.commit()
+
+    manager = HandoffManager(test_db)
+    handoff = await manager.create_handoff(
+        session_id=session.id,
+        intent="escalate",
+        engineer_notes="please help",
+        user_id=test_user["user_data"]["id"],
+    )
+    await test_db.commit()
+
+    from uuid import UUID as PyUUID
+    account_id = PyUUID(test_user["user_data"]["account_id"])
+
+    queue = await escalation_bus.subscribe(account_id)
+    try:
+        with patch(
+            "app.services.handoff_manager.EmailService.send_notification_email",
+            new=AsyncMock(return_value=True),
+        ):
+            await manager.dispatch_escalation_notifications(handoff)
+
+        import asyncio
+        event = await asyncio.wait_for(queue.get(), timeout=1.0)
+        assert event["type"] == "handoff_created"
+        assert event["handoff_id"] == str(handoff.id)
+        assert event["session_id"] == str(session.id)
+        assert event["priority"] == "normal"
+    finally:
+        await escalation_bus.unsubscribe(account_id, queue)
+
+
 @pytest.mark.asyncio
 async def test_create_handoff_endpoint_dispatches_on_escalate(
     client: AsyncClient, test_user, auth_headers, test_db