fix(api): widen slugify suffix from 4 to 8 chars or add retry loop

Summary

Event slug generation uses Math.random().toString(36).slice(2, 6) — a 4-character base36 suffix. Birthday-paradox math gives a collision rate around 1-in-1.6M per name, surfacing as a generic 500 from the unique-constraint insert. Low but non-zero.

Requirements

• Widen the suffix to 8 characters (~1-in-2.8B collision per name) OR add a try/retry loop that regenerates on unique-constraint conflict
• Files affected: packages/api/src/routes/admin/events/index.ts, packages/api/src/routes/eventsSubmit.ts
• If a shared slug.ts helper lands first (#2002), apply the fix once there
• Unit test: 10,000 random slugifies of the same name produce zero duplicates

Context

Flagged in the Plan 2 review. Forms (Plan 4) will also need slug generation — fix once and reuse.

Start by reading the slug generation in packages/api/src/routes/admin/events/index.ts and packages/api/src/routes/eventsSubmit.ts, then check whether shared slug.ts from #2002 exists. Run the relevant unit tests and add coverage for 10,000 slugs from the same name; done means duplicate-free generation and no generic unique-constraint failure path.