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/migrations/0038_genres_name_lower_unique.sql

@@ -0,0 +1,45 @@
+-- Enforce case-insensitive genre uniqueness, matching `authors` and `tags`
+-- (both got `UNIQUE (lower(name))` in 0009). `genres` only had a case-SENSITIVE
+-- `name UNIQUE`, but the crawler's `sync_genres` treats genres as
+-- case-insensitive (`WHERE lower(name) = lower($1)`) and can INSERT a new row
+-- from a source string. Under a race (or across ticks) two different-cased
+-- strings — "Isekai" vs "isekai" — could both miss the pre-check and both
+-- insert, since the exact-name unique doesn't collide. That leaves duplicate
+-- genre rows for one logical genre.
+
+-- Heal any pre-existing case-variant duplicates before adding the index, so
+-- `sqlx::migrate!` can't crash on a dirty table (mirrors 0031's pre-dedup).
+-- Keep the lowest id per lower(name) as canonical and repoint manga_genres.
+UPDATE manga_genres mg
+   SET genre_id = d.keep_id
+  FROM (
+        SELECT g.id AS dup_id, k.keep_id
+          FROM genres g
+          JOIN (SELECT lower(name) AS lname, (array_agg(id ORDER BY id))[1] AS keep_id
+                  FROM genres GROUP BY lower(name)) k
+            ON lower(g.name) = k.lname AND g.id <> k.keep_id
+       ) d
+ WHERE mg.genre_id = d.dup_id
+   AND NOT EXISTS (
+        SELECT 1 FROM manga_genres x
+         WHERE x.manga_id = mg.manga_id AND x.genre_id = d.keep_id
+   );
+
+-- Drop links that would have collided with the canonical link (already present).
+DELETE FROM manga_genres mg
+ USING (
+        SELECT g.id AS dup_id
+          FROM genres g
+          JOIN (SELECT lower(name) AS lname, (array_agg(id ORDER BY id))[1] AS keep_id
+                  FROM genres GROUP BY lower(name)) k
+            ON lower(g.name) = k.lname AND g.id <> k.keep_id
+       ) d
+ WHERE mg.genre_id = d.dup_id;
+
+-- Remove the now-orphaned duplicate genre rows.
+DELETE FROM genres g
+ USING (SELECT lower(name) AS lname, (array_agg(id ORDER BY id))[1] AS keep_id
+          FROM genres GROUP BY lower(name)) k
+ WHERE lower(g.name) = k.lname AND g.id <> k.keep_id;
+
+CREATE UNIQUE INDEX genres_name_lower_uniq ON genres (lower(name));