Remove legacy user role implementation

[taobojlen]

Context & Requests for Reviewers

Follow-up from #1061. Now that all orgs have DB-backed roles and permissions, we don't need the fallback code, and we don't need the legacy role column in the database. Instead, we can always use role_id -- an actual foreign key that refers to the role in the database.

Tests

I ran this locally, using an org first created on main, and tested that I can click around (+ edit permissions) without issues.

(Optional) Rollout Plan

Because this removes a GraphQL field, any live users may get some errors in their browsers until they refresh their page. I think this is acceptable for this self-hosted software. The alternative is to deprecate the field and stop using it, and then only later (when everyone has updated to that version), then fully remove it. I don't think the juice is worth the squeeze there.

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 the CHANGELOG.md and related docs?

• If you changed server/models/**/{ContentTypeModel,ActionModel,RuleModel,PolicyModel}.ts:
  Did you update the corresponding history tables and their triggers?

• 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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[julietshen]

LGTM!

the DROP COLUMN part might need to be called out in the rollout note to make it super clear that the DB change is forward-only.