test(frontend): pin abort-chain + signal pass-through contracts (0.87.16)

Three 0.87.9 follow-ups:

1. hooks.server.ts: two new tests covering the
   already-aborted-at-entry and mid-flight client-disconnect branches
   of the abort chain. Mutation-confirmed.

2. lib/api/admin.ts: two new tests pinning the
   getAnalysisMangaCoverage(init?.signal) pass-through and the
   AbortError-rejection propagation the dashboard's debounce-race
   handler relies on. Mutation-confirmed.

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

backend/Cargo.lock

@@ -1517,7 +1517,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.87.15"
+version = "0.87.16"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.87.15"
+version = "0.87.16"
 edition = "2021"
 default-run = "mangalord"