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.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.128.24"
+version = "0.128.25"
 dependencies = [
  "anyhow",
  "argon2",