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.

.gitignore

@@ -10,3 +10,7 @@ main
 
 # Database dumps
 *_dump.sql
+
+# Generated with `just openapi`
+/openapi.json
+/openapi-3.0.json