bugfix: subquery-wrap pending chapters query so DISTINCT + ORDER BY agree (0.26.1)

PG rejects `SELECT DISTINCT c.id, c.manga_id, cs.source_url ... ORDER BY
c.manga_id, c.created_at` because the ORDER BY references a column not in
the DISTINCT projection. Wrap the DISTINCT in a subquery (which includes
created_at) and apply the ORDER BY in the outer SELECT.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.26.0"
+version = "0.26.1"
 edition = "2021"
 default-run = "mangalord"