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/tests/crawler_sync.rs

@@ -674,6 +674,23 @@ async fn arbitrary_genres_from_source_get_inserted(pool: PgPool) {
     assert_eq!(webtoons_count.0, 1, "case-insensitive lookup reuses the existing row");
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn genres_reject_case_variant_duplicates_at_the_db(pool: PgPool) {
+    // The sequential pre-check in sync_genres dedups the common case, but two
+    // concurrent inserts could each miss it. The lower(name) unique index (0038)
+    // is the race backstop: a case variant of an existing genre must be rejected
+    // by the DB itself. "Action" is seeded by 0009.
+    let dup = sqlx::query("INSERT INTO genres (name) VALUES ('action')")
+        .execute(&pool)
+        .await;
+    let err = dup.expect_err("a case-variant of a seeded genre must violate the unique index");
+    let msg = err.to_string();
+    assert!(
+        msg.contains("genres_name_lower_uniq") || msg.contains("unique") || msg.contains("duplicate"),
+        "expected a unique-violation, got: {msg}"
+    );
+}
+
 /// User-attached tags (rows with non-NULL `added_by` in `manga_tags`)
 /// must survive a crawler upsert. The crawler owns source-attached tags
 /// (added_by IS NULL); user attachments are owned by the user who made