fix: remove event-loop-blocking cold start; unify profiler to cursor-based incremental

Cold start fetched all logs in one bulk query then processed them in a tight
synchronous loop with no yields, blocking the asyncio event loop for seconds
on datasets of 30K+ rows. This stalled every concurrent await — including the
SSE stream generator's initial DB calls — causing the dashboard to show
INITIALIZING SENSORS indefinitely.

Changes:
- Drop _cold_start() and get_all_logs_raw(); uninitialized state now runs the
  same cursor loop as incremental, starting from last_log_id=0
- Yield to the event loop after every _BATCH_SIZE rows (asyncio.sleep(0))
- Add SSE keepalive comment as first yield so the connection flushes before
  any DB work begins
- Add Cache-Control/X-Accel-Buffering headers to StreamingResponse

decnet/web/router/stream/api_stream_events.py

@@ -40,6 +40,8 @@ async def stream_events(
         loops_since_stats = 0
         emitted_chunks = 0
         try:
+            yield ": keepalive\n\n"  # flush headers immediately; helps diagnose pre-yield hangs
+
             if last_id == 0:
                 last_id = await repo.get_max_log_id()
 
@@ -90,4 +92,11 @@ async def stream_events(
             log.exception("SSE stream error for user %s", last_event_id)
             yield f"event: error\ndata: {json.dumps({'type': 'error', 'message': 'Stream interrupted'})}\n\n"
 
-    return StreamingResponse(event_generator(), media_type="text/event-stream")
+    return StreamingResponse(
+        event_generator(),
+        media_type="text/event-stream",
+        headers={
+            "Cache-Control": "no-cache",
+            "X-Accel-Buffering": "no",
+        },
+    )