fix(slack): Clear usergroup when no accepted students have Slack IDs

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Detail bug report: View on Detail

Bug

updateSlackUserGroups keeps a Slack usergroup in sync with an event's currently-accepted students who have linked Slack IDs. When the roster became empty (e.g., all previously-added students left the program), a if (ids.length > 0) guard skipped the Slack API call entirely, so departed members stayed in the usergroup indefinitely.

There was no way for the codebase to clear a usergroup: Slack's usergroups.users.update rejects an empty users list (the guard only suppressed that error, it didn't clear the group), and no usergroups.disable/enable existed anywhere in the repo. The stale group was then @-mentioned via <!subteam^...> on student-targeted scheduled announcements, addressing users who had left the program. Introduced in 93061ab7, which wrapped the call in the guard and turned a previously-visible no_users_provided error into a silent leak.

Fix

• When the roster is empty and a usergroup exists, call slack.usergroups.disable (the only Slack API way to actually clear a group) and persist slackUserGroupId = null on the event, both in the DB and in-memory.
• Made usergroups.create lazy — only when at least one accepted student has a Slack ID — so empty events don't create placeholder groups, and the cleared id lets the group be recreated fresh when students later re-join.
• Clearing the id also makes sendScheduledAnnouncements skip the stale <!subteam^...> mention (it already gated that token on slackUserGroupId).
• Added an optional second slack parameter (defaulting to the real client) so the logic is unit-testable offline; existing single-argument callers (slackSync.ts, slackArchive.ts) are unaffected.

Testing

• Unit tests, typecheck, and the existing Slack reporting regression test all pass.
• Added an offline unit test (src/slack/updateSlackUserGroups.test.ts, following the repo's existing custom-harness convention) covering: empty-roster disable+clear, populated-roster update-only, lazy create+update, the no-op case (no group yet), accepted students without Slack IDs, multi-project flattening with null/empty slackId filtering, and the re-join recreate path.
• End-to-end against a real Postgres database (no DB server was available in the environment, so one was stood up via Docker and the schema applied with prisma db push): seeded an active event with a populated usergroup, moved all students off ACCEPTED, and confirmed usergroups.disable was called and Event.slackUserGroupId persisted as null in the real database; restoring a student recreated a fresh group with a new id. Also ran the real sendScheduledAnnouncements task against the database and confirmed a STUDENT-targeted Slack announcement posted the body with no <!subteam^...> mention when the id was cleared, and still appended the mention in the non-null contrast case.
• Could not fully verify against a real Slack workspace: no Slack bot token exists in this environment, so every real Slack API call returns invalid_auth. I confirmed the fix's usergroups.disable call is actually issued through the real @slack/web-api WebClient via the real hourly slackSync entry point (the call reaches Slack and fails only on authentication), but could not inspect the usergroup inside a real workspace to confirm Slack marks it disabled. That step requires an operator's Slack workspace token with the usergroups:write scope.
• Lint is not a CI gate here, and the installed @typescript-eslint/parser is incompatible with the repo's TypeScript 5.x across the whole tree (reproduces on untouched files), so it could not be run.

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