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>

backend/requirements.txt

@@ -7,3 +7,6 @@ python-dotenv==1.0.1
 httpx==0.27.0
 apscheduler==3.10.4
 pydantic==2.7.1
+python-jose[cryptography]==3.3.0
+bcrypt==4.1.3
+email-validator==2.1.1