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/adminconsole/views.go

@@ -68,14 +68,18 @@ type UserDetailView struct {
 	TimeZone               string
 	Guest                  bool
 	NotificationsInAppOnly bool
-	HintBalance            int
-	CreatedAt              string
-	HasStats               bool
-	Stats                  StatsRow
-	Identities             []IdentityRow
-	Games                  []GameRow
-	TelegramID             string
-	ConnectorEnabled       bool
+	PaidAccount            bool
+	// MergedInto is the primary account id when this account has been retired by a
+	// merge (Stage 11), or empty for a live account.
+	MergedInto       string
+	HintBalance      int
+	CreatedAt        string
+	HasStats         bool
+	Stats            StatsRow
+	Identities       []IdentityRow
+	Games            []GameRow
+	TelegramID       string
+	ConnectorEnabled bool
 }
 
 // StatsRow is an account's lifetime statistics.