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

gateway/internal/backendclient/api.go

@@ -33,6 +33,20 @@ type ProfileResp struct {
 	NotificationsInAppOnly bool   `json:"notifications_in_app_only"`
 }
 
+// LinkResultResp is the result of an account link/merge step (Stage 11). Status is
+// "linked", "merge_required" (the secondary_* fields summarise the other account) or
+// "merged". Token is a switched-session token (a guest initiator's durable
+// counterpart won); Profile is the surviving/active account's profile.
+type LinkResultResp struct {
+	Status           string       `json:"status"`
+	SecondaryUserID  string       `json:"secondary_user_id"`
+	SecondaryName    string       `json:"secondary_display_name"`
+	SecondaryGames   int          `json:"secondary_games"`
+	SecondaryFriends int          `json:"secondary_friends"`
+	Token            string       `json:"token"`
+	Profile          *ProfileResp `json:"profile"`
+}
+
 // TileJSON is one placed tile, used in both play requests and move responses.
 type TileJSON struct {
 	Row    int    `json:"row"`