Hide role-irrelevant Schedule kind filters; show "Lightning Talks"

[Kitkatnik]

Summary

• Volunteer kind filter is now hidden from Attendees and logged-out users — only Volunteers and Admins see it.
• Reception kind filter is hidden everywhere (including admin) since it's not actionable.
• Remaining filter buttons are sorted alphabetically by display label, and lightning renders as "Lightning Talks" in both the filter button and the badge on event cards.
• Display logic is centralized in a new ScheduleHelper (kind_display_label, visible_filter_kinds); the DB enum, by_kind scope, and controller param validation are unchanged so URLs like ?kind=lightning still work.

Test plan

• Logged out at /schedule: filter shows All, Activity, Community, Embassy, Lightning Talks, Meal, Talk (no Volunteer, no Reception)
• As Attendee: same as logged out
• As Volunteer: same set + Volunteer at the end
• As Admin: same as Volunteer; /admin/schedule_items shows the same filter list and the Kind column shows "Lightning Talks" for Lightning rows
• On /schedule, any Lightning event card's badge reads "Lightning Talks" and brand styling on .item-badge--lightning is preserved
• Clicking "Lightning Talks" filters URL to /schedule?kind=lightning and only Lightning items appear

[railway-app]

🚅 Deployed to the ruby-embassy-pr-20 environment in ruby-embassy

Service	Status	Web	Updated (UTC)
ruby-embassy	✅ Success (View Logs)	Web	Apr 26, 2026 at 6:20 am