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

tests/test_service_isolation.py

@@ -212,8 +212,7 @@ class TestAttackerWorkerIsolation:
         from decnet.profiler.worker import _WorkerState, _incremental_update
 
         mock_repo = MagicMock()
-        mock_repo.get_all_logs_raw = AsyncMock(return_value=[])
-        mock_repo.get_max_log_id = AsyncMock(return_value=0)
+        mock_repo.get_logs_after_id = AsyncMock(return_value=[])
         mock_repo.set_state = AsyncMock()
 
         state = _WorkerState()