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_ratelimit.go

@@ -23,7 +23,7 @@ type rateLimitReportEntry struct {
 }
 
 // handleRateLimitReport ingests one gateway rejection report into the rate
-// watch — the admin console's throttled view and the high-rate auto-flag (R3).
+// watch — the admin console's throttled view and the high-rate auto-flag.
 // Internal, gateway-only: like sessions/resolve it trusts the network segment.
 // Malformed individual entries are skipped by the watch itself.
 func (s *Server) handleRateLimitReport(c *gin.Context) {