Workstream C: convert commands/storage to a folder module, extract thumbnail/people/series-prefs commands

- Move commands/storage.rs to commands/storage/mod.rs.
- Extract three cohesive, self-contained clusters:
  - storage/thumbnails.rs: thumbnail cache + image-URL commands.
  - storage/people.rs: person/cast metadata cache commands.
  - storage/series_prefs.rs: per-series preferred audio track commands.
- Re-exported via pub use so command names stay at commands::storage::*;
  invoke_handler unchanged. mod.rs shrinks from 1979 to 1500 lines, all tests pass.

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

@@ -0,0 +1,92 @@
+//! Per-series preferred audio track commands.
+
+use std::sync::Arc;
+use serde::{Deserialize, Serialize};
+use tauri::State;
+
+use super::DatabaseWrapper;
+use crate::storage::db_service::{DatabaseService, Query, QueryParam};
+
+
+/// Audio track preference for a series
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SeriesAudioPreference {
+    pub series_id: String,
+    pub audio_track_display_title: Option<String>,
+    pub audio_track_language: Option<String>,
+    pub audio_track_index: Option<i32>,
+}
+
+/// Save user's preferred audio track for a series
+#[tauri::command]
+pub async fn storage_save_series_audio_preference(
+    db: State<'_, DatabaseWrapper>,
+    user_id: String,
+    series_id: String,
+    server_id: String,
+    audio_track_display_title: Option<String>,
+    audio_track_language: Option<String>,
+    audio_track_index: Option<i32>,
+) -> Result<(), String> {
+    let db_service = {
+        let database = db.0.lock().map_err(|e| e.to_string())?;
+        Arc::new(database.service())
+    };
+
+    let query = Query::with_params(
+        "INSERT INTO series_audio_preferences
+            (user_id, series_id, server_id, audio_track_display_title, audio_track_language, audio_track_index, updated_at)
+         VALUES (?, ?, ?, ?, ?, ?, datetime('now'))
+         ON CONFLICT (user_id, series_id, server_id) DO UPDATE SET
+            audio_track_display_title = excluded.audio_track_display_title,
+            audio_track_language = excluded.audio_track_language,
+            audio_track_index = excluded.audio_track_index,
+            updated_at = datetime('now')",
+        vec![
+            QueryParam::String(user_id),
+            QueryParam::String(series_id),
+            QueryParam::String(server_id),
+            audio_track_display_title.map(QueryParam::String).unwrap_or(QueryParam::Null),
+            audio_track_language.map(QueryParam::String).unwrap_or(QueryParam::Null),
+            audio_track_index.map(|i| QueryParam::Int64(i as i64)).unwrap_or(QueryParam::Null),
+        ],
+    );
+
+    db_service.execute(query).await.map_err(|e| e.to_string())?;
+    Ok(())
+}
+
+/// Get user's preferred audio track for a series
+#[tauri::command]
+pub async fn storage_get_series_audio_preference(
+    db: State<'_, DatabaseWrapper>,
+    user_id: String,
+    series_id: String,
+) -> Result<Option<SeriesAudioPreference>, String> {
+    let db_service = {
+        let database = db.0.lock().map_err(|e| e.to_string())?;
+        Arc::new(database.service())
+    };
+
+    let query = Query::with_params(
+        "SELECT series_id, audio_track_display_title, audio_track_language, audio_track_index
+         FROM series_audio_preferences
+         WHERE user_id = ? AND series_id = ?",
+        vec![QueryParam::String(user_id), QueryParam::String(series_id)],
+    );
+
+    let preference = db_service
+        .query_optional(query, |row| {
+            Ok(SeriesAudioPreference {
+                series_id: row.get(0)?,
+                audio_track_display_title: row.get(1)?,
+                audio_track_language: row.get(2)?,
+                audio_track_index: row.get(3)?,
+            })
+        })
+        .await
+        .map_err(|e| e.to_string())?;
+
+    Ok(preference)
+}