Add a command to write the OpenAPI spec without a server

`just openapi [dir]` (go run ./cmd/openapi) writes openapi.json (3.1) and openapi-3.0.json without a database or configuration. A test checks it matches the spec the server serves.

justfile

@@ -15,6 +15,10 @@ sqlc-generate:
 migrate-create name:
 	@migrate create -ext sql -dir internal/db/migrations -seq {{name}}
 
+# Write openapi.json and openapi-3.0.json without starting the server, e.g. `just openapi ../MusicPlayer/api`
+openapi out=".":
+	@go run ./cmd/openapi -out {{out}}
+
 build: sqlc-generate templ-build
 	@echo "Building..."
 	@go build -o main cmd/main.go