Refactor backend database connection, migration, seed

backend/migrations/20251017064937_00001_create_users_table.sql

@@ -0,0 +1,14 @@
+-- +goose Up
+-- +goose StatementBegin
+CREATE TABLE IF NOT EXISTS users (
+    id       INTEGER PRIMARY KEY AUTOINCREMENT,
+    username TEXT NOT NULL UNIQUE,
+    password TEXT NOT NULL,
+    role     INTEGER NOT NULL DEFAULT 2
+)
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+DROP TABLE IF EXISTS users;
+-- +goose StatementEnd