feat(account): unlink provider + change-email edges (backend + gateway)

Unlink: POST /user/link/unlink (telegram|vk) via account.RemoveIdentity, refusing
the last identity; email is never unlinked. New fbs LinkUnlinkRequest + gateway
link.unlink op, returning the refreshed profile.

Change-email: purposeChange confirm-codes (RequestChangeCode/ConfirmChange) that
atomically replace the account's confirmed email (account.replaceEmailIdentity);
a new address owned by another account is refused without disclosure, never merged.
The one-tap deeplink handles purposeChange too. Reuses the LinkEmail* fbs tables;
gateway link.email.change.{request,confirm} ops + backendclient methods; branded
ru/en change-email copy.

pkg/fbs/scrabble.fbs

@@ -501,6 +501,12 @@ table LinkTelegramRequest {
   data:string;
 }
 
+// LinkUnlinkRequest detaches a platform identity (kind = "telegram" | "vk") from the
+// caller's account; email is never unlinked (it is changed).
+table LinkUnlinkRequest {
+  kind:string;
+}
+
 // LinkResult is the unified result of a confirm or merge step. status is "linked"
 // (bound to the caller), "merge_required" (the identity belongs to another account —
 // the secondary_* fields summarise it for the irreversible confirmation), or

pkg/fbs/scrabblefb/LinkUnlinkRequest.go

@@ -0,0 +1,60 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type LinkUnlinkRequest struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsLinkUnlinkRequest(buf []byte, offset flatbuffers.UOffsetT) *LinkUnlinkRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &LinkUnlinkRequest{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishLinkUnlinkRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsLinkUnlinkRequest(buf []byte, offset flatbuffers.UOffsetT) *LinkUnlinkRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &LinkUnlinkRequest{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedLinkUnlinkRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *LinkUnlinkRequest) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *LinkUnlinkRequest) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *LinkUnlinkRequest) Kind() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func LinkUnlinkRequestStart(builder *flatbuffers.Builder) {
+	builder.StartObject(1)
+}
+func LinkUnlinkRequestAddKind(builder *flatbuffers.Builder, kind flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(kind), 0)
+}
+func LinkUnlinkRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}