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.

breadarr-shared/src/dto.rs

@@ -143,6 +143,7 @@ pub struct HealthDetail {
     pub last_import_cycle: Option<CycleInfo>,
     pub last_search_cycle: Option<CycleInfo>,
     pub last_upgrade_cycle: Option<CycleInfo>,
+    pub last_transcode_cycle: Option<CycleInfo>,
     pub search_halted: bool,
 }