feat(cron): refactor cron utility with validation and metadata

- Add input validation for name, schedule expression, and handler
- Store full CronEntry metadata (handler, schedule, timezone, registeredAt)
  instead of raw job instance to support introspection
- Add JSDoc typedefs for CronEntry and improve all function docs
- Use globalThis symbol store to survive Next.js hot-reload
- Remove verbose per-run info logs to reduce noise
- Replace `||` with `??` for runOnInit default to handle falsy correctly
- Fix stop/stopAll to access `entry.job` from new storage structure

tsup.config.js

@@ -15,6 +15,7 @@ export default defineConfig([
       'src/features/admin/components/index.js',
       'src/core/api/index.js',
       'src/core/api/route-handler.js',
+      'src/core/cron/index.js',
       'src/core/database/index.js',
       'src/cli/database.js',
       'src/core/email/index.js',
@@ -41,7 +42,7 @@ export default defineConfig([
     splitting: false,
     sourcemap: false,
     clean: true,
-    external: ['react', 'react-dom', 'next', 'pg', 'dotenv', 'dotenv/config', 'resend', '@react-email/components', 'node-cron', 'readline', 'crypto', 'url', 'fs', 'path', 'net', 'dns', 'tls', '@zen/core/api', '@zen/core/database', '@zen/core/email', '@zen/core/email/templates', '@zen/core/storage', '@zen/core/toast', '@zen/core/modules/actions', '@zen/core/modules/storage', '@aws-sdk/client-s3', '@aws-sdk/s3-request-presigner'],
+    external: ['react', 'react-dom', 'next', 'pg', 'dotenv', 'dotenv/config', 'resend', '@react-email/components', 'node-cron', 'readline', 'crypto', 'url', 'fs', 'path', 'net', 'dns', 'tls', '@zen/core/api', '@zen/core/cron', '@zen/core/database', '@zen/core/email', '@zen/core/email/templates', '@zen/core/storage', '@zen/core/toast', '@zen/core/modules/actions', '@zen/core/modules/storage', '@aws-sdk/client-s3', '@aws-sdk/s3-request-presigner'],
     noExternal: [],
     bundle: true,
     banner: {