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/profile_test.go

@@ -12,7 +12,7 @@ import (
 
 // TestUpdateProfileValidation checks that bad fields are rejected before any
 // database access, so a nil-backed Store is enough to exercise the guards. It also
-// confirms UpdateProfile wires the Stage 8 validators (name format, away window,
+// confirms UpdateProfile wires the validators (name format, away window,
 // offset/IANA timezone), not just their unit tests in validate_test.go.
 func TestUpdateProfileValidation(t *testing.T) {
 	s := &Store{}