fix(server): avoid false restore failures on large database imports

[ErasmusAndre]

Description

I ran into an issue while migrating an Immich database backup where the database import completed, but the post-restore health check timed out, the restore flow rolled the database back, and the frontend showed null instead of a useful error message.

These changes increase the maintenance restore API health check timeout from 20 seconds to 180 seconds, and reject health check failures as Error objects instead of raw strings.

In practice, 20 seconds is too short for this validation path on larger restores. Increasing the timeout should prevent these unnecessary restore failures, and rejecting with Error objects keeps the failure shape consistent for downstream handling.

How Has This Been Tested?

• Reviewed the restore health check code path in server/src/maintenance/maintenance-health.repository.ts
• Confirmed the timeout used during restore validation is now 180 seconds instead of 20 seconds
• Full local test suite
• Reproduced and verified end-to-end in a local Immich dev environment

Checklist:

• I have carefully read CONTRIBUTING.md
• 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.

LLM-assisted investigation and draft wording. The final code change was reviewed by AI before submission.

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