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/cmd/backend/main.go

@@ -18,10 +18,12 @@ import (
 	"go.uber.org/zap"
 
 	"scrabble/backend/internal/account"
+	"scrabble/backend/internal/accountmerge"
 	"scrabble/backend/internal/config"
 	"scrabble/backend/internal/connector"
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
+	"scrabble/backend/internal/link"
 	"scrabble/backend/internal/lobby"
 	"scrabble/backend/internal/notify"
 	"scrabble/backend/internal/postgres"
@@ -138,6 +140,9 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	// groups) for the handlers to come.
 	mailer := newMailer(cfg.SMTP, logger)
 	emails := account.NewEmailService(accounts, mailer)
+	// Stage 11 account linking & merge: the orchestrator over the account, merge and
+	// session layers. Wired to the /api/v1/user/link REST surface below.
+	links := link.NewService(emails, accounts, accountmerge.NewMerger(db), sessions)
 	socialSvc := social.NewService(social.NewStore(db), accounts, games)
 	socialSvc.SetNotifier(hub)
 
@@ -170,6 +175,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		Matchmaker:    matchmaker,
 		Invitations:   invitations,
 		Emails:        emails,
+		Links:         links,
 		Registry:      registry,
 		DictDir:       cfg.Game.DictDir,
 		Connector:     conn,