Restructured project

- backend moved to backend directory
- added and initialized frontend with vue
- moved infrastructure files to infra directory

backend/migrations/000001_create_users.up.sql

@@ -0,0 +1,13 @@
+CREATE TABLE users
+(
+    id            BIGSERIAL PRIMARY KEY,
+    telegram_id   BIGINT UNIQUE NOT NULL,
+    username      TEXT,
+    first_name    TEXT          NOT NULL,
+    last_name     TEXT,
+    language_code TEXT,
+    created_at    TIMESTAMP     NOT NULL DEFAULT NOW(),
+    updated_at    TIMESTAMP     NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_users_telegram_id ON users (telegram_id);