feat: establish Go service and database foundation

.env.example

@@ -0,0 +1,12 @@
+# Local development only. Copy to .env and replace secrets; .env is ignored.
+APP_ENV=development
+HTTP_ADDR=127.0.0.1:8080
+DATABASE_URL=postgresql://postgres:change-me@localhost:5432/zarinpal?sslmode=disable
+REDIS_ADDR=localhost:6379
+REDIS_PASSWORD=
+IMPORT_PATH=data/incoming/challenge_data.csv.gz
+DEMO_USERNAME=merchant-0001
+DEMO_PASSWORD=replace-with-at-least-12-characters
+SESSION_SECRET=replace-with-at-least-32-random-characters
+FRONTEND_ORIGIN=http://localhost:5173
+COOKIE_SECURE=false