Workstream E (backend): wire tauri-specta — annotate commands, derive Type, generate-ready Builder

- Add #[specta::specta] to all 201 #[tauri::command] functions.
- Derive specta::Type on all IPC DTOs (repository/types, settings, player/storage/
  download command DTOs, player enums, jellyfin SessionInfo/NowPlayingItem/PlayState,
  ThumbnailCacheStats, DownloadInfo, CacheConfig, etc.).
- Replace tauri::generate_handler! with a tauri_specta::Builder + collect_commands!
  in lib.rs (exports bindings.ts in debug builds).

Two contract changes required by specta constraints (frontend migration follows):
- specta caps command arity at 10 args: download_item_and_start / download_item /
  download_video now take a single request struct (params bundled, body unchanged
  via destructuring).
- specta can't parse split serde rename_all: SessionInfo/NowPlayingItem/PlayState
  switched to rename_all = "PascalCase" (Jellyfin deserialization preserved; these
  now serialize PascalCase to the frontend).

cargo check --lib is clean (0 errors). Frontend migration to bindings.ts is the next step.

src-tauri/src/commands/storage/thumbnails.rs

@@ -15,6 +15,7 @@ use crate::thumbnail::{ThumbnailCacheStats, ThumbnailWorker};
 /// Get cached thumbnail path, returns None if not cached
 /// Also updates last_accessed timestamp for LRU tracking
 #[tauri::command]
+#[specta::specta]
 pub async fn thumbnail_get_cached(
     db: State<'_, DatabaseWrapper>,
     thumbnail_cache: State<'_, ThumbnailCacheWrapper>,
@@ -38,6 +39,7 @@ pub async fn thumbnail_get_cached(
 /// Download and save a thumbnail to cache
 /// Returns the local file path on success
 #[tauri::command]
+#[specta::specta]
 pub async fn thumbnail_save(
     db: State<'_, DatabaseWrapper>,
     thumbnail_cache: State<'_, ThumbnailCacheWrapper>,
@@ -65,6 +67,7 @@ pub async fn thumbnail_save(
 
 /// Get thumbnail cache statistics
 #[tauri::command]
+#[specta::specta]
 pub async fn thumbnail_get_stats(
     db: State<'_, DatabaseWrapper>,
     thumbnail_cache: State<'_, ThumbnailCacheWrapper>,
@@ -87,6 +90,7 @@ pub async fn thumbnail_get_stats(
 
 /// Set thumbnail cache storage limit in bytes
 #[tauri::command]
+#[specta::specta]
 pub async fn thumbnail_set_limit(
     db: State<'_, DatabaseWrapper>,
     thumbnail_cache: State<'_, ThumbnailCacheWrapper>,
@@ -102,6 +106,7 @@ pub async fn thumbnail_set_limit(
 
 /// Clear all cached thumbnails
 #[tauri::command]
+#[specta::specta]
 pub async fn thumbnail_clear_cache(
     db: State<'_, DatabaseWrapper>,
     thumbnail_cache: State<'_, ThumbnailCacheWrapper>,
@@ -116,6 +121,7 @@ pub async fn thumbnail_clear_cache(
 
 /// Delete cached thumbnails for a specific item
 #[tauri::command]
+#[specta::specta]
 pub async fn thumbnail_delete_item(
     db: State<'_, DatabaseWrapper>,
     thumbnail_cache: State<'_, ThumbnailCacheWrapper>,
@@ -149,7 +155,7 @@ fn image_semaphore() -> &'static Semaphore {
 }
 
 /// Request to get an image URL (with caching)
-#[derive(Debug, Deserialize)]
+#[derive(specta::Type, Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct GetImageRequest {
     pub item_id: String,
@@ -165,6 +171,7 @@ pub struct GetImageRequest {
 /// Get image as base64 data URL, caching if not already cached
 /// This extends the thumbnail system to serve all images through Rust with automatic caching
 #[tauri::command]
+#[specta::specta]
 pub async fn image_get_url(
     repository_manager: State<'_, RepositoryManagerWrapper>,
     thumbnail_cache: State<'_, ThumbnailCacheWrapper>,