Workstream C: convert commands/download to a folder module, extract pinning and smart-cache commands

- Move commands/download.rs to commands/download/mod.rs.
- Extract pin/unpin/is_pinned into download/pinning.rs.
- Extract smart-cache stats/config + album recommendation commands into
  download/smart_cache.rs.
- Re-exported via pub use so command names stay at commands::download::*;
  invoke_handler unchanged, all tests pass.

src-tauri/src/commands/download/pinning.rs

@@ -0,0 +1,63 @@
+//! Pinning commands - protect an item's cached metadata from cache clearing.
+
+use std::sync::Arc;
+use tauri::State;
+
+use crate::commands::DatabaseWrapper;
+use crate::storage::db_service::{DatabaseService, Query, QueryParam};
+
+/// Pin an item's metadata (protects from cache clear)
+#[tauri::command]
+pub async fn pin_item(db: State<'_, DatabaseWrapper>, item_id: String) -> 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(
+        "UPDATE items SET is_pinned = 1 WHERE id = ?",
+        vec![QueryParam::String(item_id)],
+    );
+
+    db_service.execute(query).await.map_err(|e| e.to_string())?;
+    Ok(())
+}
+
+/// Unpin an item's metadata
+#[tauri::command]
+pub async fn unpin_item(db: State<'_, DatabaseWrapper>, item_id: String) -> 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(
+        "UPDATE items SET is_pinned = 0 WHERE id = ?",
+        vec![QueryParam::String(item_id)],
+    );
+
+    db_service.execute(query).await.map_err(|e| e.to_string())?;
+    Ok(())
+}
+
+/// Check if an item is pinned
+#[tauri::command]
+pub async fn is_item_pinned(db: State<'_, DatabaseWrapper>, item_id: String) -> Result<bool, 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 COALESCE(is_pinned, 0) FROM items WHERE id = ?",
+        vec![QueryParam::String(item_id)],
+    );
+
+    let is_pinned: i32 = db_service
+        .query_optional(query, |row| row.get(0))
+        .await
+        .map_err(|e| e.to_string())?
+        .unwrap_or(0);
+
+    Ok(is_pinned == 1)
+}