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/commands/storage/thumbnails.rs

@@ -1,9 +1,11 @@
 //! Thumbnail cache and image-URL commands.
+//!
+//! TRACES: UR-007 | JA-028 | DR-016
 
-use std::sync::{Arc, OnceLock};
-use tokio::sync::Semaphore;
 use serde::Deserialize;
+use std::sync::{Arc, OnceLock};
 use tauri::State;
+use tokio::sync::Semaphore;
 
 use super::{DatabaseWrapper, ThumbnailCacheWrapper};
 use crate::commands::repository::RepositoryManagerWrapper;
@@ -11,7 +13,6 @@ use crate::repository::types::{ImageOptions, ImageType};
 use crate::repository::MediaRepository;
 use crate::thumbnail::{ThumbnailCacheStats, ThumbnailWorker};
 
-
 /// Get cached thumbnail path, returns None if not cached
 /// Also updates last_accessed timestamp for LRU tracking
 #[tauri::command]
@@ -28,7 +29,8 @@ pub async fn thumbnail_get_cached(
         Arc::new(database.service())
     };
 
-    let result = thumbnail_cache.0
+    let result = thumbnail_cache
+        .0
         .get_cached_path(db_service, &item_id, &image_type, &tag)
         .await
         .map(|p| p.to_string_lossy().to_string());
@@ -61,7 +63,10 @@ pub async fn thumbnail_save(
         Arc::new(database.service())
     };
 
-    let path = thumbnail_cache.0.save_thumbnail(db_service, &item_id, &image_type, &tag, &data, None, None).await?;
+    let path = thumbnail_cache
+        .0
+        .save_thumbnail(db_service, &item_id, &image_type, &tag, &data, None, None)
+        .await?;
     Ok(path.to_string_lossy().to_string())
 }
 
@@ -179,7 +184,7 @@ pub async fn image_get_url(
     repository_handle: String,
     request: GetImageRequest,
 ) -> Result<String, String> {
-    use base64::{Engine as _, engine::general_purpose::STANDARD as BASE64};
+    use base64::{engine::general_purpose::STANDARD as BASE64, Engine as _};
     use std::fs;
 
     let tag = request.tag.as_deref().unwrap_or("default");
@@ -191,14 +196,18 @@ pub async fn image_get_url(
     };
 
     // Check cache first
-    if let Some(cached_path) = thumbnail_cache.0.get_cached_path(
-        db_service.clone(),
-        &request.item_id,
-        &request.image_type,
-        tag,
-    ).await {
-        let image_data = fs::read(&cached_path)
-            .map_err(|e| format!("Failed to read cached image: {}", e))?;
+    if let Some(cached_path) = thumbnail_cache
+        .0
+        .get_cached_path(
+            db_service.clone(),
+            &request.item_id,
+            &request.image_type,
+            tag,
+        )
+        .await
+    {
+        let image_data =
+            fs::read(&cached_path).map_err(|e| format!("Failed to read cached image: {}", e))?;
         let base64_data = BASE64.encode(&image_data);
         let mime_type = mime_from_ext(cached_path.extension().and_then(|s| s.to_str()));
         return Ok(format!("data:{};base64,{}", mime_type, base64_data));
@@ -206,10 +215,14 @@ pub async fn image_get_url(
 
     // Not cached — fetch from server and cache.
     // Acquire semaphore to limit concurrent downloads (prevents connection pool starvation).
-    let _permit = image_semaphore().acquire().await
+    let _permit = image_semaphore()
+        .acquire()
+        .await
         .map_err(|_| "Image download semaphore closed".to_string())?;
 
-    let repository = repository_manager.0.get(&repository_handle)
+    let repository = repository_manager
+        .0
+        .get(&repository_handle)
         .ok_or_else(|| "Repository not found - user may need to log in".to_string())?;
 
     let image_type_enum = match request.image_type.as_str() {
@@ -229,18 +242,23 @@ pub async fn image_get_url(
     };
 
     let server_url = repository.get_image_url(&request.item_id, image_type_enum, Some(options));
-    let image_data = repository.download_bytes(&server_url).await
+    let image_data = repository
+        .download_bytes(&server_url)
+        .await
         .map_err(|e| format!("Failed to download image: {}", e))?;
 
-    let cached_path = thumbnail_cache.0.save_thumbnail(
-        db_service,
-        &request.item_id,
-        &request.image_type,
-        tag,
-        &image_data,
-        request.max_width.map(|w| w as i32),
-        request.max_height.map(|h| h as i32),
-    ).await?;
+    let cached_path = thumbnail_cache
+        .0
+        .save_thumbnail(
+            db_service,
+            &request.item_id,
+            &request.image_type,
+            tag,
+            &image_data,
+            request.max_width.map(|w| w as i32),
+            request.max_height.map(|h| h as i32),
+        )
+        .await?;
 
     let base64_data = BASE64.encode(&image_data);
     let mime_type = mime_from_ext(cached_path.extension().and_then(|s| s.to_str()));