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Adds "Upcoming photos to show" to the config UI (queue_target_len, 5-50, default 20, replacing the hardcoded QUEUE_TARGET_LEN constant). Lowering it trims the queue immediately on next page load rather than waiting for enough advances to consume the excess naturally; raising it tops back up the same way, via a new photo_queue.sync_queue_length() called from GET /api/queue.