chapter extraction via ffprobe during library sync (#5)

ffprobe module in adapter-common: parse JSON output into Vec<Chapter>,
should_probe_chapters gate (movie OR >45min), graceful failure.
Add set_chapters/set_role setters to MediaItem.

crates/adapters/adapter-common/Cargo.toml

@@ -11,3 +11,4 @@ uuid = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 tracing = { workspace = true }
+tokio = { workspace = true }