feat: implement GET /v1/devices/:id/relay for relay endpoint discovery
Adds device relay endpoint discovery so provisioned devices know where
to connect. Authenticated with device_token; returns relay_url,
websocket_url, and optional iroh_endpoint from config.
- Add IrohConfig.endpoint (Option<String>) to common config
- Add require_device_token auth helper scoped to device id + token hash
- Implement GET /v1/devices/:id/relay handler with 8 tests
- Add seed_device test helper to test_utils
- Add Bruno collection entry (seq 25)