feat: add structured GET /api/v1/connections response

[Koziar]

Related PR

beamlynx/beamlynx-ui#29

Summary

Replaces the raw GET /api/v1/connections response with a structured payload the UI can use to list live database connections, show human-readable labels, and know which connection Pine currently has selected.

Changes

API — GET /api/v1/connections

Response shape (wrapped in result, consistent with GET /api/v1/connection):

{
  "result": {
    "version": "0.35.0",
    "selected-connection-id": "localhost:5432",
    "connections": [
      {
        "id": "localhost:5432",
        "label": "localhost:5432 · myapp_dev"
      }
    ]
  }
}

• selected-connection-id — value of @db/connection-id (Pine’s currently selected connection), or null when no connection is selected.
• connections — all registered connection pools.
• id — existing connection handle (host:port), unchanged from prior behavior.
• label — display string {host}:{port} · {dbname}, derived from the pool JDBC URL.

API surface

Only change to an existing route: GET /api/v1/connections — same path, new response shape (was raw @pools map; now structured result as above).

docker/init-pine.sh calls it once for debug logging and does not parse the body.