feat: save user theme preference

[calebmcquaid]

Context & Requests for Reviewers

This is the backend piece for the dark mode issue (#365). This allows a user to store a personal preference of light/dark mode. This adds a new column in the DB, a GQL enum, and persistence in the userManagementService.

Tests

Unit tests added, lints and builds clean.

(Optional) Rollout Plan

This is a stacked PR and should land with the others in the stack.

Checklist

Only check items that apply to this PR; leave the rest unchecked.

• If you changed anything user-facing (i.e. user interface or APIs):
  Did you update related docs?

• If the change is notable (refer to Keep a Changelog conventions):
  Did you update CHANGELOG.md?

• If you changed db/src/scripts/** and used CREATE TABLE, ADD COLUMN, or ALTER COLUMN:
  Are as many columns marked NOT NULL as possible? If some columns can sometimes be null depending on other columns, are there CHECK constraints capturing those relationships, and are these also reflected using unions in the associated Kysely types?

• If you added a new signal in server/services/signalsService/signals/**:
  Did you classify every error case as a permanent error (SignalPermanentError, no retry) or a normal error (retryable)? Any case where the signal can't determine a score should be a SignalPermanentError.

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Summary by cubic

Saves per-user color scheme preferences so the client can persist theme choices instead of resetting on reload.

Users can now set a theme preference (SYSTEM, LIGHT, or DARK) via the new setThemePreference mutation. The value is stored in user_interface_settings and exposed as themePreference on user interface settings; null means unset, and the client falls back to light.

Migration

• Adds a nullable theme_preference column with a CHECK constraint; existing users default to null.

^{Written for commit 9ef3930. Summary will update on new commits.}

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