webrtc: emit RFC 9261 §5.2 Link headers on WHIP 201 Publish response (issue #21)

app/webrtc/whip_handler.go

@@ -174,6 +174,13 @@ func (h *WHIPHandler) Publish(c echo.Context) error {
 
 	h.recordRequest("publish", id, http.StatusCreated, t0)
 
+	// RFC 9261 §5.2: emit one Link header per configured ICE server so
+	// that the publisher (OBS, browser, GStreamer, etc.) can discover
+	// STUN/TURN without a separate signalling round-trip — symmetric
+	// with the WHEP Subscribe Link header added in issue #19.
+	for _, uri := range h.sub.ICEServerURIs() {
+		c.Response().Header().Add("Link", "<"+uri+">; rel=\"ice-server\"")
+	}
 	c.Response().Header().Set("Content-Type", "application/sdp")
 	c.Response().Header().Set("Location", "/whip/"+id+"/"+rid)
 	c.Response().Header().Set("ETag", `"`+rid+`"`)
@@ -229,6 +236,13 @@ func (h *WHIPHandler) Unpublish(c echo.Context) error {
 // TrickleIngest handles PATCH /whip/:id/:resource — adds ICE candidates
 // from a trickle-ice-sdpfrag body.
 //
+// The body format follows draft-ietf-wish-whip §5: one or more
+// "a=candidate:…" lines in application/trickle-ice-sdpfrag. Each
+// matching line is forwarded directly to the underlying PeerConnection.
+// An empty body or a body with no candidate lines is a no-op (clients
+// signal end-of-candidates via an a=end-of-candidates line, which
+// AddICECandidate correctly ignores at the Pion level).
+//
 // @Summary Trickle ICE candidates for a WHIP publish session
 // @Tags v16.16.0
 // @ID webrtc-3-whip-trickle