fix(server): store null instead of empty string for user oauthId

[deveshkolte]

Description

When a user signs up via email (without OAuth), the database now stores NULL instead of an empty string "" for the OAuth ID column.

This was already marked with a TODO comment in the admin service:

// TODO replace  with null

Changes:

• DB column: nullable with default NULL; migration updates existing rows
• Response schemas (UserAdminResponse, SyncUserV1): allows null
• Admin service: unlinkAll now sets to null instead of empty string
• Auth service: unlink OAuth now sets to null instead of empty string
• TypeScript types (UserAdmin, AuthUserEvent): oauthId is string | null

Fixes #28832 (user.oauthId column)

Checklist:

• I have carefully read CONTRIBUTING.md
• I have performed a self-review of my own code
• I have no unrelated changes in the PR.
• I have followed naming conventions/patterns in the surrounding code
• I have written tests for new code (if applicable)

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

None.

[immich-push-o-matic]

Label error. Requires exactly 1 of: changelog:.*. Found: 🗄️server. A maintainer will add the required label.

[timonrieger]

@deveshkolte please make sure to follow the guidelines made in the issue's pinned comment (and also apply it to your other PRs for that issue)