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/002_push_subscriptions.sql

@@ -0,0 +1,13 @@
+-- 002_push_subscriptions.sql
+-- Push notification subscriptions storage
+
+CREATE TABLE IF NOT EXISTS push_subscriptions (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    endpoint TEXT NOT NULL UNIQUE,
+    p256dh TEXT NOT NULL,
+    auth TEXT NOT NULL,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Index for faster lookups by endpoint
+CREATE INDEX IF NOT EXISTS idx_push_subscriptions_endpoint ON push_subscriptions(endpoint);