feat(db): add migrations system and push subscriptions persistence

- Add sqlx migration system with migrations/ directory
- Create 001_init.sql and 002_push_subscriptions.sql migration files
- Move from manual CREATE TABLE to version-controlled migrations
- Add push_subscriptions table with DB persistence
- PushSubscriptionStore now loads from DB on startup
- Add save/remove/get methods for push subscriptions in db.rs
- Move VAPID keys to .env file (with fallback to hardcoded values)
- Delete old vibetorrent.db and recreate with migrations

backend/.env

@@ -0,0 +1,5 @@
+# VAPID Keys for Push Notifications
+# Generate new keys for production using: cargo run --bin web-push --features web-push -- generate-vapid-keys
+VAPID_PUBLIC_KEY=BEdPj6XQR7MGzM28Nev9wokF5upHoydNDahouJbQ9ZdBJpEFAN1iNfANSEvY0ItasNY5zcvvqN_tjUt64Rfd0gU
+VAPID_PRIVATE_KEY=aUcCYJ7kUd9UClCaWwad0IVgbYJ6svwl19MjSX7GH10
+VAPID_EMAIL=mailto:admin@vibetorrent.app