Add module loading, variable definitions, output format selection,
aggregate mode, type checking, and code formatting to mq-web-api.

New endpoints:
- POST /api/check — type-check a query via mq-check (always 200,
  errors are data); returns syntax and type errors with source positions
- POST /api/format — format mq code via mq-formatter; supports
  indent_width, sort_imports, sort_functions, sort_fields options

Extended ApiRequest fields (all optional, backward-compatible):
- modules: load named builtin modules before query execution
- args: define string variables accessible in queries
- output_format: markdown (default) | html | text | json | none
- aggregate: prepend `nodes | import "section" |` prefix (CLI -A flag)

Deps: add mq-check, mq-formatter, mq-hir; enable json feature on mq-markdown