Add system board templates seeded at startup

Four templates (Kanban, Simple, Sprint, Bug Tracking) with tenantId=null
are auto-seeded on first startup if none exist. Available to all tenants
in the board creation dropdown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks.server.ts

@@ -1,8 +1,11 @@
 import type { Handle } from '@sveltejs/kit';
-import { resolveTenant } from '$lib/server/tenant.js';
+import { resolveTenant, seedSystemTemplates } from '$lib/server/tenant.js';
 import { validateSession, sessionCookieName } from '$lib/server/auth.js';
 import { logger } from '$lib/server/logger.js';
 
+// One-time seed on startup
+seedSystemTemplates().catch((e) => logger.error({ err: e }, 'Failed to seed system templates'));
+
 export const handle: Handle = async ({ event, resolve }) => {
 	const start = Date.now();
 	// 1. Resolve tenant from hostname