admin: add account spam restriction (join/message gate)

Adds a narrower spam sanction alongside the existing account freeze: a
restricted account keeps every existing membership and conversation, but
cannot join new channels/groups (public join or invite link) and cannot
start a new conversation with a non-contact. Reachable both as a standalone
admin action and as a decision on a reported user's moderation case, with
the same idempotent-supersession and appeal wiring freeze already has.

internal/rpc/errors.go

@@ -128,6 +128,14 @@ func mediaEmptyErr() error { return tgerr.New(400, "MEDIA_EMPTY") }
 func frozenMethodInvalidErr() error      { return tgerr.New(420, "FROZEN_METHOD_INVALID") }
 func frozenParticipantMissingErr() error { return tgerr.New(400, "FROZEN_PARTICIPANT_MISSING") }
 
+// restrictedMethodInvalidErr covers a spam-restricted account attempting to
+// join a new channel/group (public join or private invite link).
+func restrictedMethodInvalidErr() error { return tgerr.New(420, "USER_RESTRICTED") }
+
+// restrictedNoncontactErr covers a spam-restricted account attempting to
+// start a new conversation with a peer that is not in its contacts.
+func restrictedNoncontactErr() error { return tgerr.New(403, "USER_RESTRICTED_NONCONTACT") }
+
 func photoInvalidErr() error { return tgerr.New(400, "PHOTO_INVALID") }
 
 func stickersetInvalidErr() error { return tgerr.New(406, "STICKERSET_INVALID") }