fix: improve book matching in metadata sync with bidirectional title search

Pre-fetch all local books in one query instead of N queries per external
book. Match by volume number first, then bidirectional title containment
(external in local OR local in external). Track matched IDs to prevent
double-matching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/metadata.rs

@@ -829,48 +829,48 @@ async fn sync_books_metadata(
     let mut matched_count: i64 = 0;
     let mut book_reports: Vec<BookSyncReport> = Vec::new();
 
-    for book in &books {
+    // Pre-fetch all local books for this series to enable flexible matching
-        // Try to match with local book by volume_number first, then title
+    let local_books: Vec<(Uuid, Option<i32>, String)> = sqlx::query_as(
-        let local_book_id: Option<Uuid> = if let Some(vol) = book.volume_number {
-            sqlx::query_scalar(
         r#"
-                SELECT id FROM books
+        SELECT id, volume, title FROM books
         WHERE library_id = $1
           AND COALESCE(NULLIF(series, ''), 'unclassified') = $2
-                  AND volume = $3
-                LIMIT 1
         "#,
     )
     .bind(library_id)
     .bind(series_name)
-            .bind(vol)
+    .fetch_all(&state.pool)
-            .fetch_optional(&state.pool)
+    .await?;
-            .await?
+
+    // Track which local books have already been matched to avoid double-matching
+    let mut matched_local_ids = std::collections::HashSet::new();
+
+    for book in &books {
+        // Strategy 1: Match by volume number
+        let mut local_book_id: Option<Uuid> = if let Some(vol) = book.volume_number {
+            local_books
+                .iter()
+                .find(|(id, v, _)| *v == Some(vol) && !matched_local_ids.contains(id))
+                .map(|(id, _, _)| *id)
         } else {
             None
         };
 
-        let local_book_id = match local_book_id {
+        // Strategy 2: External title contained in local title or vice-versa (case-insensitive)
-            Some(id) => Some(id),
+        if local_book_id.is_none() {
-            None => {
+            let ext_title_lower = book.title.to_lowercase();
-                // Try matching by title
+            local_book_id = local_books.iter().find(|(id, _, local_title)| {
-                let pattern = format!("%{}%", book.title);
+                if matched_local_ids.contains(id) {
-                sqlx::query_scalar(
+                    return false;
-                    r#"
+                }
-                    SELECT id FROM books
+                let local_lower = local_title.to_lowercase();
-                    WHERE library_id = $1
+                local_lower.contains(&ext_title_lower) || ext_title_lower.contains(&local_lower)
-                      AND COALESCE(NULLIF(series, ''), 'unclassified') = $2
+            }).map(|(id, _, _)| *id);
-                      AND title ILIKE $3
+        }
-                    LIMIT 1
+
-                    "#,
+        if let Some(id) = local_book_id {
-                )
+            matched_local_ids.insert(id);
-                .bind(library_id)
-                .bind(series_name)
-                .bind(&pattern)
-                .fetch_optional(&state.pool)
-                .await?
         }
-        };
 
         sqlx::query(
             r#"