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/cache.rs

@@ -36,7 +36,7 @@ impl Default for CacheConfig {
             album_affinity_enabled: true,
             album_affinity_threshold: 3,
             storage_limit: 10 * 1024 * 1024 * 1024, // 10GB
-            wifi_only: false, // Allow preloading on any connection by default
+            wifi_only: false,                       // Allow preloading on any connection by default
         }
     }
 }
@@ -225,7 +225,10 @@ impl SmartCache {
                 "DELETE FROM downloads WHERE id = ?",
                 vec![QueryParam::Int64(id)],
             );
-            db_service.execute(delete_query).await.map_err(|e| e.to_string())?;
+            db_service
+                .execute(delete_query)
+                .await
+                .map_err(|e| e.to_string())?;
 
             freed += size as u64;
         }

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();

src-tauri/src/download/mod.rs

@@ -11,8 +11,8 @@ pub mod events;
 pub mod worker;
 
 use crate::utils::lock::MutexSafe;
-use std::path::PathBuf;
 use std::collections::HashSet;
+use std::path::PathBuf;
 use std::sync::{Arc, Mutex};
 
 pub use worker::DownloadWorker;

src-tauri/src/download/worker.rs

@@ -60,7 +60,11 @@ impl DownloadWorker {
     }
 
     /// Attempt a single download
-    async fn try_download<F>(&self, task: &DownloadTask, on_progress: &F) -> Result<DownloadResult, DownloadError>
+    async fn try_download<F>(
+        &self,
+        task: &DownloadTask,
+        on_progress: &F,
+    ) -> Result<DownloadResult, DownloadError>
     where
         F: Fn(u64, Option<u64>) + Send + Sync,
     {
@@ -74,10 +78,7 @@ impl DownloadWorker {
         // Check for partial download
         let temp_path = task.target_path.with_extension("part");
         let existing_bytes = if temp_path.exists() {
-            fs::metadata(&temp_path)
-                .await
-                .map(|m| m.len())
-                .unwrap_or(0)
+            fs::metadata(&temp_path).await.map(|m| m.len()).unwrap_or(0)
         } else {
             0
         };
@@ -105,14 +106,17 @@ impl DownloadWorker {
             .get(reqwest::header::CONTENT_LENGTH)
             .and_then(|v| v.to_str().ok())
             .and_then(|v| v.parse::<u64>().ok())
-            .map(|len| if existing_bytes > 0 { len + existing_bytes } else { len });
+            .map(|len| {
+                if existing_bytes > 0 {
+                    len + existing_bytes
+                } else {
+                    len
+                }
+            });
 
         // Open file for appending
         let mut file = if existing_bytes > 0 {
-            fs::OpenOptions::new()
-                .append(true)
-                .open(&temp_path)
-                .await
+            fs::OpenOptions::new().append(true).open(&temp_path).await
         } else {
             fs::File::create(&temp_path).await
         }
@@ -206,9 +210,18 @@ mod tests {
 
     #[test]
     fn test_exponential_backoff() {
-        assert_eq!(DownloadWorker::exponential_backoff(1), Duration::from_secs(5));
-        assert_eq!(DownloadWorker::exponential_backoff(2), Duration::from_secs(15));
-        assert_eq!(DownloadWorker::exponential_backoff(3), Duration::from_secs(45));
+        assert_eq!(
+            DownloadWorker::exponential_backoff(1),
+            Duration::from_secs(5)
+        );
+        assert_eq!(
+            DownloadWorker::exponential_backoff(2),
+            Duration::from_secs(15)
+        );
+        assert_eq!(
+            DownloadWorker::exponential_backoff(3),
+            Duration::from_secs(45)
+        );
     }
 
     #[test]