fix: set correct Content-Type for Prometheus metrics endpoint

[opyh]

🖊️ Description

💡 What? Why? How?

Fixed the Prometheus metrics endpoint returning incorrect Content-Type: text/html header, which caused Prometheus scrapes to fail with:

received unsupported Content-Type "text/html" and no fallback_scrape_protocol specified for target

Changes:

1. Added explicit Content-Type: text/plain; version=0.0.4; charset=utf-8 header (Prometheus standard)
2. Fixed typo in join delimiter: "/n" → "\n" (forward slash vs backslash)
3. Updated OpenAPI spec to declare text/plain instead of application/json

Root cause: Nitro.js defaults to text/html when no Content-Type header is explicitly set.

📎 Related Ticket

Fixes TargetDown alert for a11yscore-metrics in production monitoring.

⚠️ Creation checklist

• The automated tests are passing.
• My code is self-documenting
• The commit messages follow conventional commits standard

🔬 Testing instructions

After deployment, verify:

1. curl -u prometheus:$PASSWORD https://a11yscore.wheelmap.tech/a11yscore/v1/auth/metrics/prometheus -I returns Content-Type: text/plain; version=0.0.4; charset=utf-8
2. Prometheus targets show a11yscore-bullmq as healthy in the monitoring namespace