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/mod.rs

@@ -78,6 +78,7 @@ pub struct CycleStatus {
     pub last_import: Option<CycleRecord>,
     pub last_search: Option<CycleRecord>,
     pub last_upgrade: Option<CycleRecord>,
+    pub last_transcode: Option<CycleRecord>,
     /// Set once the search-driven loop's consecutive-failure backoff hits
     /// its ceiling — still ticking at max backoff underneath (self-healing
     /// if the source recovers), but worth a loud, easy-to-spot signal that