feat: accept fhirlint.yml config block as API parameter

[predictor2718]

Goal

Allow the /api/validate endpoint to accept a fhirlint.yml config block alongside the FHIR content, instead of (or in addition to) individual fields. This lets users reuse their project config directly and connects the composer to the validator.

Motivation

Currently the API takes individual fields (fhirVersion, profiles, etc.). Teams already have a fhirlint.yml in their repo and would have to re-enter all options manually. With a config parameter they can paste the YAML once and validate.

API

POST /api/validate

{
  "content": "{ \"resourceType\": \"Patient\", ... }",
  "config": "fhir-version: 4.0.1\nprofile:\n  - kbv-basis\nno-terminology-server: true\n"
}

Rules:

• config is parsed as fhirlint.yml on the server
• Individual fields (e.g. fhirVersion) override the config if both are present
• Invalid config returns a 400 with validation errors (same format as /api/config/check)

Composer integration

The "Open in Validator →" button in the composer (see composer issue) passes the generated YAML as the config parameter — one click from composer to validator with all settings pre-applied.

Notes

• Config parsing reuses the same validation logic as fhirlint config check (feat: add fhirlint config check command to validate fhirlint.yml fhirlint#94)