Stage 11: account linking & merge (email + Telegram Login Widget)

Link an email (confirm-code) or Telegram (web Login Widget) to the current
account; if the identity already has its own account, merge the two into the
one in use (the current account is primary, except a guest initiator whose
durable counterpart wins). The merge runs in one transaction
(internal/accountmerge): stats + hint wallet summed, paid_account ORed,
identities/games/chat/complaints transferred, friends/blocks de-duplicated,
the secondary kept as a merged_into tombstone so a shared finished game's
no-cascade FKs hold; a shared active game blocks the merge.

- migration 00009: accounts.paid_account, merged_into, merged_at (+ jetgen)
- internal/link orchestrator; session.RevokeAllForAccount on merge
- connector ValidateLoginWidget RPC + loginwidget HMAC validator
- edge ops link.email.request/confirm/merge, link.telegram.confirm/merge;
  supersedes the Stage 8 email.bind.* surface (request never reveals 'taken'
  before the code is verified, so a probe cannot enumerate addresses)
- UI Profile link section + irreversible-merge dialog; Telegram web sign-in
- focused regression tests (merge core, guest inversion, active-game refusal,
  finished-shared-game kept), gateway transcode + connector + UI codec/e2e
- docs: PLAN, ARCHITECTURE 3/4/9, FUNCTIONAL(+ru), module READMEs

backend/internal/session/service.go

@@ -71,3 +71,14 @@ func (svc *Service) Revoke(ctx context.Context, token string) error {
 	svc.cache.Remove(hash)
 	return nil
 }
+
+// RevokeAllForAccount revokes every active session of accountID and evicts them
+// from the cache. The account-merge flow calls it to retire a secondary account
+// (Stage 11). It is idempotent.
+func (svc *Service) RevokeAllForAccount(ctx context.Context, accountID uuid.UUID) error {
+	if _, err := svc.store.RevokeAllForAccount(ctx, accountID, time.Now().UTC()); err != nil {
+		return err
+	}
+	svc.cache.RemoveByAccount(accountID)
+	return nil
+}