feat: Update project structure and configuration files

backend/database/migrations/20250322103243_create_user_posts.sql

@@ -0,0 +1,18 @@
+-- +goose Up
+-- +goose StatementBegin
+CREATE TABLE user_posts(
+    id SERIAL8 PRIMARY KEY,
+    created_at timestamp NOT NULL DEFAULT now(),
+    updated_at timestamp NOT NULL DEFAULT now(),
+    --
+    user_id int8 NOT NULL,
+    post_id int8 NOT NULL,
+    price int8 NOT NULL
+);
+
+-- +goose StatementEnd
+-- +goose Down
+-- +goose StatementBegin
+DROP TABLE user_posts;
+
+-- +goose StatementEnd