Support per-client DID filters for WS stream
Accept a comma-separated "dids" query param on WebSocket connect and
store per-client DID lists. Apply the DID filter to the initial
paginated query (using drizzle-orm inArray) and to broadcastEvent so
clients only receive matching events. Also fix bufferedAmount typing in
backpressure checks, reduce PAGE_DELAY_MS to 3ms, and remove the
periodic heartbeat timer.