Move all slug generation on create to the DB (groups, stages, festivals)

[chiptus]

Part 2 of the split of #41 (see that issue for background). Depends on #465 (slugify non-empty fallback) — land that first.

Goal

The DB is the single owner of slug generation on insert. sets and artists already have BEFORE INSERT dedupe triggers (supabase/migrations/20260804101202_add_slug_dedupe_triggers.sql) that derive a slug via public.slugify() when none is supplied. Extend the same pattern to the remaining tables and stop the client from computing slugs on create.

Work

Migration — add the same trigger pattern (derive from name when NEW.slug is empty/NULL, advisory-lock + -2 suffix dedupe within the relevant uniqueness scope) to:

• groups
• stages (scope: per festival_edition_id, matching its unique constraint)
• festivals and festival_editions (check the actual table/constraint names and slug scoping in existing migrations, e.g. 20250808000001_add_festival_slugs.sql)

Make slug columns nullable-on-insert if they currently have NOT NULL without default in a way that blocks trigger-filled inserts (triggers run BEFORE, so likely fine — verify).

Client — remove generateSlug from the create paths so no slug is sent on insert:

• src/api/groups/useCreateGroup.ts:20
• src/api/stages/useCreateStage.ts:17
• Festival create paths in src/pages/admin/festivals/FestivalDialog.tsx / FestivalEditionManagement.tsx (the create flows only; the slug edit field on update flows is Edit views: explicit slug field instead of silent re-slugging on rename #467's scope — keep any explicit user-entered slug pass-through working, since the triggers trust a caller-supplied slug)

Acceptance

• Creating a group/stage/festival/edition without a slug gets a DB-generated, deduped, non-empty slug — including for non-Latin names.
• Creating with an explicit slug (admin festival dialogs allow one) keeps that slug.
• No generateSlug import remains in any create-path hook.
• Integration tests cover a trigger-generated slug and a dedupe collision for at least one of the new tables.