Remote Control stream events gain a configurable first-flush delay

What

The client that uploads events for Remote Control (bridge) sessions gained a new streamEventFirstFlushMs setting, alongside an existing setStreamEventFlushIntervalMs setter:

• When streamEventFirstFlushMs is set to a value greater than zero, the very first stream_event is buffered as ephemeral instead of being sent immediately, short-circuiting the normal flush-interval batching just for that first event.
• Both the flush interval and this new first-flush delay are now pushed into the client at runtime via the periodic config-apply routine, alongside the existing no-subscriber flush interval, no-subscriber upload hold, and drop-unwatched-events settings.
• The setting defaults to 0 (no delay) and is read from remote config.

Why

This gives finer control over how quickly the first event of a Remote Control session reaches the backend, which can be tuned separately from the steady-state flush interval used for events after a subscriber is actively watching.