feat(web): orchestrator events read API + SSE stream

GET /api/v1/orchestrator/events — paginated list with optional
kind=traffic|file filter. GET /api/v1/orchestrator/events/stream —
SSE: snapshot on connect, live forward of orchestrator.> bus events
mapped to 'traffic' / 'file' SSE event names.

Repo gains list_orchestrator_events(limit, offset, kind?, since_ts?),
count_orchestrator_events(kind?), and prune_orchestrator_events
(per_dst_cap=10000) for periodic worker-side trimming.

decnet/web/router/orchestrator/api_events.py

@@ -0,0 +1,123 @@
+"""SSE stream of orchestrator events.
+
+Subscribes to ``orchestrator.>`` for the duration of the request and
+forwards each event as a Server-Sent Event. Emits a one-shot snapshot
+on connect (latest 50 rows).
+
+Mirror of :mod:`decnet.web.router.campaigns.api_events`.
+"""
+from __future__ import annotations
+
+import asyncio
+from typing import AsyncGenerator
+
+import orjson
+from fastapi import APIRouter, Depends, Request
+from fastapi.responses import StreamingResponse
+
+from decnet.bus import topics as _topics
+from decnet.bus.app import get_app_bus
+from decnet.logging import get_logger
+from decnet.telemetry import traced as _traced
+from decnet.web.dependencies import repo, require_stream_viewer
+from decnet.web.sse_limits import sse_connection_slot
+
+log = get_logger("api.orchestrator.events")
+
+router = APIRouter()
+
+_KEEPALIVE_SECS = 15.0
+_SNAPSHOT_LIMIT = 50
+
+
+def _format_sse(event_name: str, data: dict) -> str:
+    return f"event: {event_name}\ndata: {orjson.dumps(data).decode()}\n\n"
+
+
+@router.get(
+    "/orchestrator/events/stream",
+    tags=["Orchestrator"],
+    responses={
+        200: {
+            "content": {"text/event-stream": {}},
+            "description": "SSE stream of orchestrator events",
+        },
+        401: {"description": "Could not validate credentials"},
+        403: {"description": "Insufficient permissions"},
+        429: {"description": "Per-user SSE connection cap reached"},
+    },
+)
+@_traced("api.orchestrator.events")
+async def api_orchestrator_events(
+    request: Request,
+    user: dict = Depends(require_stream_viewer),
+) -> StreamingResponse:
+    snapshot = await repo.list_orchestrator_events(
+        limit=_SNAPSHOT_LIMIT, offset=0,
+    )
+
+    async def generator() -> AsyncGenerator[str, None]:
+        async with sse_connection_slot(user["uuid"]):
+            yield ": keepalive\n\n"
+            yield _format_sse("snapshot", {"events": snapshot})
+
+            bus = await get_app_bus()
+            if bus is None:
+                while not await request.is_disconnected():
+                    try:
+                        await asyncio.sleep(_KEEPALIVE_SECS)
+                    except asyncio.CancelledError:
+                        break
+                    yield ": keepalive\n\n"
+                return
+
+            sub = bus.subscribe(f"{_topics.ORCHESTRATOR}.>")
+            try:
+                async with sub:
+                    sub_iter = sub.__aiter__()
+                    while True:
+                        if await request.is_disconnected():
+                            break
+                        next_task = asyncio.ensure_future(sub_iter.__anext__())
+                        try:
+                            event = await asyncio.wait_for(
+                                next_task, timeout=_KEEPALIVE_SECS,
+                            )
+                        except asyncio.TimeoutError:
+                            next_task.cancel()
+                            yield ": keepalive\n\n"
+                            continue
+                        except StopAsyncIteration:
+                            break
+                        yield _format_sse(
+                            _sse_name_for(event.topic),
+                            {
+                                "topic": event.topic,
+                                "type": event.type,
+                                "ts": event.ts,
+                                "payload": event.payload,
+                            },
+                        )
+            except asyncio.CancelledError:
+                pass
+            except Exception:
+                log.exception("orchestrator events stream crashed")
+                yield _format_sse("error", {"message": "Stream interrupted"})
+
+    return StreamingResponse(
+        generator(),
+        media_type="text/event-stream",
+        headers={
+            "Cache-Control": "no-cache",
+            "X-Accel-Buffering": "no",
+        },
+    )
+
+
+def _sse_name_for(topic: str) -> str:
+    """``orchestrator.traffic.<uuid>`` → ``traffic``;
+    ``orchestrator.file.<uuid>`` → ``file``."""
+    parts = topic.split(".", 2)
+    if len(parts) >= 2 and parts[0] == _topics.ORCHESTRATOR:
+        return parts[1]
+    return topic