Add GET /v1/registry endpoint to expose registry URL

[findolor]

Goal

Expose the configured REGISTRY_URL through a public API endpoint so clients can discover which registry the server is pointed at.

Proposed Route

Method	Route	Description
GET	/v1/registry	Return the active registry URL

Response

{
  "url": "https://..."
}

Implementation

• Read REGISTRY_URL from the shared registry state introduced in [Prerequisite] Add registry-based Raindex client foundation for v1 handlers #32
• Protected by AuthenticatedKey like other v1 routes
• OpenAPI annotated under the existing v1 tag
• Tracing/instrumentation consistent with other handlers

Dependencies

• Blocked by [Prerequisite] Add registry-based Raindex client foundation for v1 handlers #32 — registry-based Raindex client foundation (provides REGISTRY_URL managed state)

Related

• [Prerequisite] Add registry-based Raindex client foundation for v1 handlers #32 — Prerequisite: registry foundation
• Implement v1 Route Handlers #30 — Parent: v1 route handlers
• Derive token list URL from registry instead of hardcoding #42 — Token list URL derived from registry