R6(a): de-stage code, docs, READMEs; split stage6_test

Mechanical, behaviour-preserving removal of Stage N / TODO-N / phase (RN)
references from comments, doc-comments, service READMEs, the current-state docs
(ARCHITECTURE, FUNCTIONAL+_ru, TESTING, UI_DESIGN), config-file comments, and the
.fbs/.proto schema comments. PLAN.md / PRERELEASE.md / CLAUDE.md keep the stage
history.

- Rename the only stage-named identifiers: registerStage8 -> registerSocialOps,
  registerStage11 -> registerLinkOps (gateway transcode).
- Split stage6_test.go: TestEmailLoginFlow -> email_test.go,
  TestGuestAutoMatchLeavesNoStats (+ provisionGuest) -> account_test.go.
- Regenerated proto bindings (push.pb.go, telegram_grpc.pb.go) from the de-staged
  .proto comments; FB Go/TS bindings unchanged (flatc strips schema comments).

go build/vet/gofmt clean across modules; integration typecheck and pnpm check green.

backend/internal/account/userlist.go

@@ -20,7 +20,7 @@ type UserListItem struct {
 	IsGuest           bool
 	IsRobot           bool
 	// FlaggedHighRateAt is the soft high-rate marker (zero when unflagged), shown
-	// as a badge in the console list (R3).
+	// as a badge in the console list.
 	FlaggedHighRateAt time.Time
 	CreatedAt         time.Time
 }
@@ -105,7 +105,7 @@ type FlaggedAccount struct {
 const flaggedListCap = 200
 
 // ListFlaggedHighRate returns the accounts carrying the high-rate flag, most
-// recently flagged first (R3).
+// recently flagged first.
 func (s *Store) ListFlaggedHighRate(ctx context.Context) ([]FlaggedAccount, error) {
 	rows, err := s.db.QueryContext(ctx,
 		`SELECT account_id, display_name, flagged_high_rate_at