fix(server): properly handle HEAD requests to SSR paths

[dav-wolff]

Description

Fixes the HTTP HEAD method not working for paths that are handled by ApiService.ssr.

The HEAD method is supposed to return the same metadata as a GET request but without the body (https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Methods/HEAD).
Previously this didn't happen because the ApiService.ssr checked if the method is GET and otherwise forwards the request to the api handlers which would result in a 404. Now ApiService.ssr also checks if the method is HEAD and handles the request. The html is still passed to res.send so that express can determine the proper Content-Length.

How Has This Been Tested?

I manually tested the status and headers (besides Date) being the same for GET and HEAD requests. I didn't write a unit test as ApiService, unlike all the other services, doesn't have a spec file and isn't able to use newTestService because it doesn't extend BaseService.
I also ran pnpm test and it didn't show any regressions.

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR.
• I have confirmed that any new dependencies are strictly necessary.
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code
• All code in src/services/ uses repositories implementations for database calls, filesystem operations, etc.
• All code in src/repositories/ is pretty basic/simple and does not have any immich specific logic (that belongs in src/services/)

Please describe to which degree, if any, an LLM was used in creating this pull request.

No AI used.