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/domain/src/models/media.rs

@@ -226,6 +226,14 @@ impl MediaItem {
     pub fn chapters(&self) -> &[Chapter] {
         &self.chapters
     }
+
+    pub fn set_chapters(&mut self, chapters: Vec<Chapter>) {
+        self.chapters = chapters;
+    }
+
+    pub fn set_role(&mut self, role: MediaRole) {
+        self.role = role;
+    }
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]