feat(scouts): deliver_pending drains queue and sends scout_proposal frames

Add ScoutEngine.deliver_pending(user_id, ws) that queries status='queued'
rows, fetches metadata via the registered connector, sends scout_proposal
WS frames, and flips status to 'delivered'. Add ack_proposal(proposal_id)
that flips 'delivered' -> 'acked' (idempotent). Wire both into device_ws.py:
deliver_pending fires as a background task after device_hello + register;
scout_proposal_ack frames dispatch to ack_proposal in the message loop.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_scout_engine.py

@@ -170,3 +170,48 @@ async def test_idempotent_replay(monkeypatch):
     async with _TestSessionLocal() as session:
         rows = (await session.execute(select(ScoutTriageQueue))).scalars().all()
     assert len(rows) == 1, "Replay must not create duplicate queue rows"
+
+
+@pytest.mark.asyncio
+async def test_deliver_pending_sends_one_frame_per_queued_row(monkeypatch):
+    user_id = "00000000-0000-0000-0000-000000000003"
+    scout_id = str(uuid.uuid4())
+    now = datetime.now(tz=timezone.utc)
+
+    async with _TestSessionLocal() as session:
+        session.add(CloudScoutConfig(
+            id=scout_id, user_id=user_id, provider="gmail", name="Test",
+            data_types=[], prompt_template="", schedule_cron="0 * * * *",
+            enabled=True, auto_trash_spam=False, device_inactivity_pause_days=14,
+        ))
+        for i in range(3):
+            session.add(ScoutTriageQueue(
+                id=str(uuid.uuid4()), user_id=user_id, scout_id=scout_id,
+                source_type="gmail", source_msg_ref=f"msg-{i}",
+                triage_verdict="relevant", status="queued",
+                triaged_at=now, expires_at=now + timedelta(days=30),
+            ))
+        await session.commit()
+
+    connector = AsyncMock()
+    connector.source_type = "gmail"
+    connector.fetch_metadata = AsyncMock(side_effect=lambda scout, ref: ItemMetadata(
+        subject=f"sub-{ref.source_msg_ref}", snippet=f"snip-{ref.source_msg_ref}",
+    ))
+    register_connector(connector)
+
+    sent = []
+    ws = AsyncMock()
+    ws.send_json = AsyncMock(side_effect=lambda payload: sent.append(payload))
+
+    engine = ScoutEngine(session_factory=_TestSessionLocal)
+    await engine.deliver_pending(uuid.UUID(user_id), ws)
+
+    assert len(sent) == 3
+    assert all(s["type"] == "scout_proposal" for s in sent)
+    subjects = {s["proposal"]["raw_subject"] for s in sent}
+    assert subjects == {"sub-msg-0", "sub-msg-1", "sub-msg-2"}
+    async with _TestSessionLocal() as session:
+        rows = (await session.execute(select(ScoutTriageQueue))).scalars().all()
+        assert all(r.status == "delivered" for r in rows)
+        assert all(r.delivered_at is not None for r in rows)