webrtc: emit RFC 9429 §4.3 Link headers on WHEP 201 response (issue #19)

app/webrtc/handler.go

@@ -204,6 +204,12 @@ func (h *Handler) Subscribe(c echo.Context) error {
 
 	h.recordRequest("subscribe", id, http.StatusCreated, t0)
 
+	// RFC 9429 §4.3: emit one Link header per configured ICE server so
+	// that the browser can discover STUN/TURN without a separate
+	// signalling round-trip.
+	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", "/whep/"+id+"/"+rid)
 	c.Response().Header().Set("ETag", `"`+rid+`"`)
@@ -405,14 +411,15 @@ func (h *Handler) tearDownStreamPeers(streamID string) {
 }
 
 // addCORS emits the response headers a browser-side WHEP player
-// expects. WHEP's Location and ETag headers must be exposed for
-// fetch() to read them across origins.
+// expects. WHEP's Location, ETag, and Link headers must be exposed
+// for fetch() to read them across origins. Link carries the ICE
+// server URIs per RFC 9429 §4.3.
 func addCORS(c echo.Context) {
 	hh := c.Response().Header()
 	hh.Set("Access-Control-Allow-Origin", "*")
 	hh.Set("Access-Control-Allow-Methods", "POST, DELETE, PATCH, OPTIONS")
 	hh.Set("Access-Control-Allow-Headers", "Content-Type, Authorization, If-Match, If-None-Match")
-	hh.Set("Access-Control-Expose-Headers", "Location, ETag")
+	hh.Set("Access-Control-Expose-Headers", "Location, ETag, Link")
 }
 
 // requireH264AndOpus does a coarse SDP scan to confirm the offer