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/migrations/001_init.sql

@@ -0,0 +1,16 @@
+-- 001_init.sql
+-- Initial schema for users and sessions
+
+CREATE TABLE IF NOT EXISTS users (
+    id INTEGER PRIMARY KEY,
+    username TEXT NOT NULL UNIQUE,
+    password_hash TEXT NOT NULL,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS sessions (
+    token TEXT PRIMARY KEY,
+    user_id INTEGER NOT NULL,
+    expires_at DATETIME NOT NULL,
+    FOREIGN KEY(user_id) REFERENCES users(id)
+);