chore(lint): propagate eslint-unicorn v72 to fork-only code

Upstream immich-app#30092 bumped eslint-plugin-unicorn to v72 and swept its own files,
disabling the noisy new unicorn/prefer-simple-condition-first rule in the
server, web and cli configs. Fork-only files were not in that sweep, so the
zero-error gate failed on five of them:

- web timeline-bucket getMonthBucket: unicorn/prefer-split-limit (the result
  is destructured to two elements, so split('-', 2) is equivalent)
- the #813 Recently Added filter specs: unicorn/prefer-string-repeat on the
  three-space blank-input fixtures

Also drops the now-unused `extname` import in asset-media.service.ts. Upstream
immich-app#30024 replaced its only call site with getFilenameExtension(); tsc does not
flag it but eslint --max-warnings 0 does.

Verified: server lint/tsc/tsc6 exit 0, web lint 0 errors, and the two touched
spec files pass (44 tests). The 8 remaining web warnings are unused
eslint-disable directives in upstream-owned files, left for upstream.

Author: Deeds67