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/server/server.go

@@ -22,6 +22,7 @@ import (
 	"scrabble/backend/internal/connector"
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
+	"scrabble/backend/internal/link"
 	"scrabble/backend/internal/lobby"
 	"scrabble/backend/internal/session"
 	"scrabble/backend/internal/social"
@@ -58,6 +59,9 @@ type Deps struct {
 	Matchmaker  *lobby.Matchmaker
 	Invitations *lobby.InvitationService
 	Emails      *account.EmailService
+	// Links drives account linking & merge (Stage 11): the /api/v1/user/link
+	// endpoints. A nil Links disables them.
+	Links *link.Service
 	// Registry holds the resident dictionaries; the admin console (Stage 10) reads
 	// its versions and hot-reloads new ones. DictDir is the dictionary directory a
 	// reload reads a version subdirectory from. A nil Registry disables the console.
@@ -85,6 +89,7 @@ type Server struct {
 	matchmaker  *lobby.Matchmaker
 	invitations *lobby.InvitationService
 	emails      *account.EmailService
+	links       *link.Service
 	registry    *engine.Registry
 	dictDir     string
 	connector   *connector.Client
@@ -124,6 +129,7 @@ func New(addr string, deps Deps) *Server {
 		matchmaker:    deps.Matchmaker,
 		invitations:   deps.Invitations,
 		emails:        deps.Emails,
+		links:         deps.Links,
 		registry:      deps.Registry,
 		dictDir:       deps.DictDir,
 		connector:     deps.Connector,