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>

app/scouts/engine.py

@@ -132,6 +132,61 @@ class ScoutEngine:
                 ref.source_msg_ref,
             )
 
+    async def deliver_pending(self, user_id: uuid.UUID, ws) -> None:
+        """Drain status='queued' rows for user, send scout_proposal WS frames, flip to 'delivered'."""
+        from app.scouts.connectors.base import ItemRef  # noqa: PLC0415
+        async with self._session_factory() as session:
+            rows = (await session.execute(
+                select(ScoutTriageQueue).where(
+                    ScoutTriageQueue.user_id == str(user_id),
+                    ScoutTriageQueue.status == "queued",
+                )
+            )).scalars().all()
+
+            for row in rows:
+                try:
+                    connector = get_connector(row.source_type)
+                except KeyError:
+                    logger.warning("deliver_pending: no connector for %s", row.source_type)
+                    continue
+                scout = await session.get(CloudScoutConfig, row.scout_id)
+                if scout is None:
+                    continue
+                try:
+                    meta = await connector.fetch_metadata(scout, ItemRef(source_msg_ref=row.source_msg_ref))
+                except Exception:
+                    logger.exception("deliver_pending: fetch_metadata failed")
+                    continue
+
+                payload = {
+                    "type": "scout_proposal",
+                    "proposal": {
+                        "id": row.id,
+                        "scout_id": row.scout_id,
+                        "source_type": row.source_type,
+                        "source_msg_ref": row.source_msg_ref,
+                        "raw_subject": meta.subject,
+                        "raw_snippet": meta.snippet,
+                        "category": "unprocessed",
+                        "payload": None,
+                    },
+                }
+                await ws.send_json(payload)
+                row.status = "delivered"
+                row.delivered_at = datetime.now(tz=timezone.utc)
+
+            await session.commit()
+
+    async def ack_proposal(self, proposal_id: str) -> None:
+        """Flip a delivered proposal to acked. Idempotent — no-op if already acked."""
+        async with self._session_factory() as session:
+            row = await session.get(ScoutTriageQueue, proposal_id)
+            if row is None:
+                return
+            row.status = "acked"
+            row.acked_at = datetime.now(tz=timezone.utc)
+            await session.commit()
+
     async def _triage_llm(self, scout: CloudScoutConfig, content: ItemContent) -> TriageVerdict:
         """Stub — real implementation in Task 24."""
         raise NotImplementedError("Real triage LLM call lands in Task 24")