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/db/repository.py

@@ -111,11 +111,6 @@ class BaseRepository(ABC):
         """Store a specific state entry by key."""
         pass
 
-    @abstractmethod
-    async def get_all_logs_raw(self) -> list[dict[str, Any]]:
-        """Retrieve all log rows with fields needed by the attacker profile worker."""
-        pass
-
     @abstractmethod
     async def get_max_log_id(self) -> int:
         """Return the highest log ID, or 0 if the table is empty."""