Handle affiliate approval notification failures

[morganschp]

Fixes #203.
Fixes #163.

Summary

• treat affiliate application approval/rejection notifications as best-effort after the status update succeeds
• reject malformed or non-object affiliate application status request bodies before offer/application update or notification work
• log notification insert errors or thrown failures instead of returning a false 500 to the seller UI
• add route regression coverage for malformed JSON, non-object JSON, and a failed notification insert

uGig task

Submitted for the active uGig repo testing task: https://ugig.net/gigs/4741218f-a723-46bb-82cb-6516120331ae

No payout details included; those can be provided after acceptance.

Verification

• ./node_modules/.bin/vitest run 'src/app/api/affiliates/offers/[id]/applications/route.test.ts'
• ./node_modules/.bin/eslint 'src/app/api/affiliates/offers/[id]/applications/route.ts' 'src/app/api/affiliates/offers/[id]/applications/route.test.ts'
• ./node_modules/.bin/prettier --check 'src/app/api/affiliates/offers/[id]/applications/route.ts' 'src/app/api/affiliates/offers/[id]/applications/route.test.ts'
• ./node_modules/.bin/tsc --noEmit --pretty false
• git diff --check -- 'src/app/api/affiliates/offers/[id]/applications/route.ts' 'src/app/api/affiliates/offers/[id]/applications/route.test.ts'