Add multi-user authentication with JWT

- Users table with email/bcrypt-hashed password; register and login via /auth/ endpoints
- JWT tokens (30-day expiry) stored in localStorage; all API routes require Bearer auth
- All data (varieties, batches, settings, notification logs) scoped to the authenticated user
- Login/register screen overlays the app; sidebar shows user email and logout button
- Scheduler sends daily ntfy summaries for every configured user
- DB schema rewritten for multi-user; SECRET_KEY added to env

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

.env.example

@@ -1,3 +1,4 @@
 MYSQL_ROOT_PASSWORD=sproutly_root_secret
 MYSQL_USER=sproutly
 MYSQL_PASSWORD=sproutly_secret
+SECRET_KEY=your-secret-key-change-this