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/server/handlers_admin_console.go

@@ -560,7 +560,7 @@ func (s *Server) consolePostBroadcast(c *gin.Context) {
 
 // consoleThrottled renders the rate-limit observability page: the recent
 // gateway-reported throttle episodes (in-memory, reset on a backend restart)
-// and the accounts currently carrying the soft high-rate flag (R3).
+// and the accounts currently carrying the soft high-rate flag.
 func (s *Server) consoleThrottled(c *gin.Context) {
 	ctx := c.Request.Context()
 	var view adminconsole.ThrottledView
@@ -595,7 +595,7 @@ func (s *Server) consoleThrottled(c *gin.Context) {
 }
 
 // consoleClearHighRateFlag clears the soft high-rate marker — the operator's
-// reversible review action (R3).
+// reversible review action.
 func (s *Server) consoleClearHighRateFlag(c *gin.Context) {
 	id, ok := s.consoleUUID(c, "/_gm/users")
 	if !ok {