feat(push): Add MSC4174 web push

src/api/client/capabilities.rs

@@ -13,6 +13,7 @@ use ruma::{
 	},
 	assign,
 };
+use serde_json::json;
 
 use crate::Ruma;
 
@@ -55,5 +56,10 @@ pub(crate) async fn get_capabilities_route(
 		assign!(ProfileFieldsCapability::new(true), { disallowed: Some(services.oidc.restricted_profile_fields()) }),
 	);
 
+	capabilities.set(
+		"org.matrix.msc4174.webpush",
+		json!({ "enabled": true, "vapid": services.pusher.webpush_vapid_public_key() }),
+	)?;
+
 	Ok(get_capabilities::v3::Response::new(capabilities))
 }