feat: refactor to service-based WebRTC signaling endpoints

BREAKING CHANGE: Replace offer-based endpoints with service-based signaling

- Add POST /services/:uuid/answer
- Add GET /services/:uuid/answer
- Add POST /services/:uuid/ice-candidates
- Add GET /services/:uuid/ice-candidates
- Remove all /offers/* endpoints (POST /offers, GET /offers/mine, etc.)
- Server auto-detects peer's offer when offerId is omitted
- Update README with new service-based API documentation
- Bump version to 0.4.0

This change simplifies the API by focusing on services rather than individual offers.
WebRTC signaling (answer/ICE) now operates at the service level, with automatic
offer detection when needed.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/rondevu-server",
-  "version": "0.3.0",
+  "version": "0.4.0",
   "description": "DNS-like WebRTC signaling server with username claiming and service discovery",
   "main": "dist/index.js",
   "scripts": {