fix: make the genre dedup migration collision-proof (0038)

Review caught a boot-crash risk: 0038's repoint-UPDATE could set two
non-canonical variant links of one manga to the same canonical id in a single
statement (NOT EXISTS sees the pre-statement snapshot) -> manga_genres PK
violation -> migration rollback -> startup failure. Replace with INSERT ... SELECT
DISTINCT ... ON CONFLICT DO NOTHING (collision-proof) then delete variants. Amended
in place (0038 is unpushed). Tested with the exact collision scenario.

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

backend/migrations/0038_genres_name_lower_unique.sql

@@ -9,32 +9,31 @@
 
 -- 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
-   );
+-- Canonical = lowest id per lower(name).
 
--- Drop links that would have collided with the canonical link (already present).
+-- 1. Ensure every manga linked to any case-variant also has the canonical link.
+--    INSERT ... ON CONFLICT DO NOTHING is collision-proof: SELECT DISTINCT
+--    collapses multiple variants of one manga to a single canonical row, and the
+--    ON CONFLICT absorbs a canonical link that already exists. (A prior
+--    UPDATE-repoint could set two non-canonical rows of the SAME manga to the
+--    same canonical id in a single statement and violate the manga_genres PK,
+--    rolling the migration back and wedging startup.)
+INSERT INTO manga_genres (manga_id, genre_id)
+SELECT DISTINCT mg.manga_id, k.keep_id
+  FROM manga_genres mg
+  JOIN genres g ON mg.genre_id = g.id
+  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
+ WHERE g.id <> k.keep_id
+ON CONFLICT (manga_id, genre_id) DO NOTHING;
+
+-- 2. Every non-canonical link now has a canonical sibling — drop the variants.
 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;
+ USING genres g,
+       (SELECT lower(name) AS lname, (array_agg(id ORDER BY id))[1] AS keep_id
+          FROM genres GROUP BY lower(name)) k
+ WHERE mg.genre_id = g.id AND lower(g.name) = k.lname AND g.id <> k.keep_id;
 
 -- Remove the now-orphaned duplicate genre rows.
 DELETE FROM genres g