Add optional app reference header to OpenAI gateway service

Summary

Add an optional app-reference header to the OpenAI gateway service of devplace.net. The header accepts a free-form string (max 30 characters, slug-valid characters only, no spaces). Default value is default. Track usage statistics per app reference, indexed for fast retrieval, to enable server-side cost attribution and transparency across all client apps.

Steps to Reproduce

1. Not provided.

Expected Behaviour

• The OpenAI gateway service accepts an optional app-reference header.
• The header value is validated: max 30 characters, slug-valid characters only (no spaces).
• If the header is absent, the default value default is used.
• Usage statistics (e.g., number of inferences, costs) are tracked per app reference and indexed for fast querying.
• All client apps within devplace.net that use AI are assigned a consistent app reference following the naming convention devplace-[app/service/whatever]-v-1-0-0.
• The /docs endpoint is updated to document the new header and strongly recommend its use.

Actual Behaviour

Not provided.

Environment

Not provided.

Reported by retoor via DevPlace.