feat: migrate setup visibility from boolean to three-tier system

Replace isPublic boolean with visibility enum (private/link/public) across
the full stack. Add shares table to schema for future share link support.
Update all services, routes, schemas, hooks, components, and tests.

Plan: 32-01 (Setup Sharing System - Schema Migration)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/helpers/db.ts

@@ -20,6 +20,7 @@ async function getOrCreateDb(): Promise<Db> {
 
 // Truncation order respects foreign keys (children first)
 const TRUNCATE_TABLES = [
+	"shares",
 	"setup_items",
 	"setups",
 	"thread_candidates",