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_session_handoffs_api.py

@@ -113,6 +113,49 @@ async def test_claim_blocked_for_viewer_role(
     assert "engineer" in claim_resp.json()["detail"].lower()
 
 
+@pytest.mark.asyncio
+async def test_escalations_stream_blocked_for_viewer(
+    client: AsyncClient, test_user, auth_headers, test_db
+):
+    """SSE stream is role-gated to engineer-or-admin (matches queue/claim)."""
+    user_id = PyUUID(test_user["user_data"]["id"])
+    user = (
+        await test_db.execute(select(User).where(User.id == user_id))
+    ).scalar_one()
+    user.account_role = "viewer"
+    await test_db.commit()
+
+    resp = await client.get(
+        "/api/v1/ai-sessions/escalations/stream", headers=auth_headers
+    )
+    assert resp.status_code == 403
+
+
+@pytest.mark.asyncio
+async def test_escalations_stream_returns_sse_content_type(
+    client: AsyncClient, test_user, auth_headers, test_db
+):
+    """Engineer/owner can open the SSE stream and gets text/event-stream
+    plus an initial `ready` event. Read just enough bytes to confirm the
+    handshake — the full pub/sub flow is covered by the bus + dispatcher
+    tests separately."""
+    async with client.stream(
+        "GET",
+        "/api/v1/ai-sessions/escalations/stream",
+        headers=auth_headers,
+    ) as resp:
+        assert resp.status_code == 200
+        assert resp.headers["content-type"].startswith("text/event-stream")
+        # First chunk must contain the ready event.
+        first = b""
+        async for chunk in resp.aiter_bytes():
+            first += chunk
+            if b"event: ready" in first and b"\n\n" in first:
+                break
+        assert b"event: ready" in first
+        assert b'"account_id"' in first
+
+
 @pytest.mark.asyncio
 async def test_claim_allowed_for_engineer_role(
     client: AsyncClient, test_user, auth_headers, test_db