Add Accept header validation to all API endpoints

[sebiboga]

Context

Currently none of the API endpoints (v1/random, v1/empty, v1/cleanjobs) validate the Accept header. While this doesn't cause functional issues, it means:

• Clients sending Accept: text/html or malformed Accept headers get JSON back anyway (which is correct behavior currently)
• No content negotiation exists for future format extensions

Suggestion

Add optional validation of Accept: application/json header at the start of each endpoint. If present and not application/json, return a 406 Not Acceptable response:

{
  "error": "Not Acceptable - only application/json is supported"
}

Endpoints affected

• GET /v1/random/
• DELETE /v1/empty/
• DELETE /v1/cleanjobs/

Notes

• This should accept wildcard */* and application/* as valid
• No Accept header should default to JSON (backward compatible)