Background-audio handoff for video + repository/player refactor

Hand video playback off to a native audio-only stream when the app is
backgrounded or locked, with no on-device video decode (UR-040). Adds
player_enter/exit_background_audio commands, an audio-only stream URL
for video items across the repository layer, and the frontend handoff
state machine wired into VideoPlayer. Includes accompanying
repository/offline/player refactoring and regenerates the traceability
matrix.

src-tauri/src/download/events.rs

@@ -8,16 +8,10 @@ use serde::{Deserialize, Serialize};
 pub enum DownloadEvent {
     /// Download has been queued
     #[serde(rename_all = "camelCase")]
-    Queued {
-        download_id: i64,
-        item_id: String,
-    },
+    Queued { download_id: i64, item_id: String },
     /// Download has started
     #[serde(rename_all = "camelCase")]
-    Started {
-        download_id: i64,
-        item_id: String,
-    },
+    Started { download_id: i64, item_id: String },
     /// Download progress update
     #[serde(rename_all = "camelCase")]
     Progress {
@@ -43,16 +37,10 @@ pub enum DownloadEvent {
     },
     /// Download paused
     #[serde(rename_all = "camelCase")]
-    Paused {
-        download_id: i64,
-        item_id: String,
-    },
+    Paused { download_id: i64, item_id: String },
     /// Download cancelled
     #[serde(rename_all = "camelCase")]
-    Cancelled {
-        download_id: i64,
-        item_id: String,
-    },
+    Cancelled { download_id: i64, item_id: String },
 }
 
 #[cfg(test)]
@@ -98,9 +86,21 @@ mod tests {
         let json = serde_json::to_string(&event).unwrap();
         assert!(json.contains("\"type\":\"completed\""));
         // Verify camelCase field names
-        assert!(json.contains("\"downloadId\":42"), "Expected downloadId (camelCase), got: {}", json);
-        assert!(json.contains("\"itemId\":\"song456\""), "Expected itemId (camelCase), got: {}", json);
-        assert!(json.contains("\"filePath\":"), "Expected filePath (camelCase), got: {}", json);
+        assert!(
+            json.contains("\"downloadId\":42"),
+            "Expected downloadId (camelCase), got: {}",
+            json
+        );
+        assert!(
+            json.contains("\"itemId\":\"song456\""),
+            "Expected itemId (camelCase), got: {}",
+            json
+        );
+        assert!(
+            json.contains("\"filePath\":"),
+            "Expected filePath (camelCase), got: {}",
+            json
+        );
 
         // Verify roundtrip
         let deserialized: DownloadEvent = serde_json::from_str(&json).unwrap();