fix: enforce case-insensitive genre uniqueness

genres had only case-sensitive name UNIQUE while authors/tags use
UNIQUE (lower(name)); sync_genres could race two case variants into duplicate
rows (audit DB-3). Migration 0038 pre-dedups then adds
UNIQUE (lower(name)); sync_genres upserts ON CONFLICT (lower(name)). Tested.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/repo/crawler.rs

@@ -247,10 +247,13 @@ async fn sync_genres(
         let genre_id = match existing {
             Some((id,)) => id,
             None => {
+                // Conflict on lower(name) (0038) not name, so a racing insert of
+                // a differently-cased variant resolves to the existing row
+                // instead of raising a unique violation.
                 let (id,): (Uuid,) = sqlx::query_as(
                     r#"
                     INSERT INTO genres (name) VALUES ($1)
-                    ON CONFLICT (name) DO UPDATE SET name = genres.name
+                    ON CONFLICT (lower(name)) DO UPDATE SET name = genres.name
                     RETURNING id
                     "#,
                 )