Add native GPU-accelerated AV1 transcode capability

Post-grab async background swap, upgrade_locked flag to prevent the
upgrade loop from re-inflating a locally-transcoded file, and a
transcode-library CLI backfill for the existing catalog. Bitrate model
calibrated against Silicon Valley's real HEVC bitrate, scaled by
resolution and AV1's encoding efficiency. HDR/2160p and anime excluded
from this first pass. Jellyfin session polling throttles batch
encoding back to 1 stream during active playback.

breadarrd/src/api/routes/health.rs

@@ -20,6 +20,7 @@ pub async fn health(State(state): State<AppState>) -> Json<HealthDetail> {
         last_import_cycle: status.last_import.as_ref().map(to_info),
         last_search_cycle: status.last_search.as_ref().map(to_info),
         last_upgrade_cycle: status.last_upgrade.as_ref().map(to_info),
+        last_transcode_cycle: status.last_transcode.as_ref().map(to_info),
         search_halted: status.search_halted,
     })
 }