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/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.128.24"
+version = "0.128.25"
 edition = "2021"
 default-run = "mangalord"