keyserver: cache remote users' device and cross-signing keys

POST /keys/query for a remote user who shares a room with a local user is
now answered from a cache, fetched whole with GET /user/devices/{userId}
on a miss. Incoming m.device_list_update EDUs are applied when their
prev_id matches the cached stream_id; anything else marks the copy stale
and it is fetched again in the background, and a stale copy is never
served. m.signing_key_update replaces the cached cross-signing keys. A
user's copy is dropped once no local user shares a room with them, by a
keyserver listener on the roomserver's emit stage.

Migration 0009 adds keyserver_remote_users, keyserver_remote_device_keys
and keyserver_remote_cross_signing_keys for sqlite and postgres.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/internal/federationapi/keys.go

@@ -20,7 +20,8 @@ type KeyService interface {
 	QueryLocalKeys(ctx context.Context, req federation.UserKeysQueryRequest) (federation.UserKeysQueryResponse, error)
 	ClaimLocalKeys(ctx context.Context, want map[string]map[string]string) (map[string]map[string]map[string]json.RawMessage, error)
 	UserDevices(ctx context.Context, userID string) (federation.UserDevicesResponse, error)
-	RemoteKeysChanged(ctx context.Context, userID string) error
+	RemoteDeviceListUpdate(ctx context.Context, update federation.DeviceListUpdateContent) error
+	RemoteSigningKeyUpdate(ctx context.Context, update federation.SigningKeyUpdateContent) error
 }
 
 // registerKeyRoutes adds the key endpoints, if there is a keyserver.
@@ -77,20 +78,33 @@ func (s *Server) handleUserDevices(w http.ResponseWriter, r *http.Request, _ str
 	return nil
 }
 
-// receiveKeyEDU takes in an m.device_list_update or m.signing_key_update.
-// Either says only that a user of the sending server changed their keys;
-// the keyserver passes that on to the local clients that care.
+// receiveKeyEDU takes in an m.device_list_update or m.signing_key_update:
+// a user of the sending server changed their keys. The keyserver brings
+// its cached copy of them up to date and tells the local clients that
+// care. An update about a user of another server is dropped — a server
+// speaks for its own users only.
 func (s *Server) receiveKeyEDU(ctx context.Context, origin string, edu federation.EDU) {
 	if s.keyService == nil {
 		return
 	}
-	var c struct {
-		UserID string `json:"user_id"`
-	}
-	if json.Unmarshal(edu.Content, &c) != nil || !sameServer(c.UserID, origin) {
+	var err error
+	switch edu.Type {
+	case federation.EDUDeviceListUp:
+		var c federation.DeviceListUpdateContent
+		if json.Unmarshal(edu.Content, &c) != nil || !sameServer(c.UserID, origin) {
+			return
+		}
+		err = s.keyService.RemoteDeviceListUpdate(ctx, c)
+	case federation.EDUSigningKeys:
+		var c federation.SigningKeyUpdateContent
+		if json.Unmarshal(edu.Content, &c) != nil || !sameServer(c.UserID, origin) {
+			return
+		}
+		err = s.keyService.RemoteSigningKeyUpdate(ctx, c)
+	default:
 		return
 	}
-	if err := s.keyService.RemoteKeysChanged(ctx, c.UserID); err != nil {
+	if err != nil {
 		s.log.Debug("ignored a key update EDU", "origin", origin, "type", edu.Type, "error", err)
 	}
 }