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/inttest/email_test.go

@@ -159,7 +159,7 @@ func TestUpdateProfilePersists(t *testing.T) {
 	}
 }
 
-// TestUpdateProfileOffsetTimezone checks the Stage 8 UTC-offset timezone: it is
+// TestUpdateProfileOffsetTimezone checks the UTC-offset timezone: it is
 // accepted, persisted verbatim, and resolved to the right fixed offset.
 func TestUpdateProfileOffsetTimezone(t *testing.T) {
 	ctx := context.Background()
@@ -181,3 +181,49 @@ func TestUpdateProfileOffsetTimezone(t *testing.T) {
 		t.Fatalf("ResolveZone offset = %d, want 10800", off)
 	}
 }
+
+// TestEmailLoginFlow covers the email-as-login path: a code is mailed to
+// a new address, verifying it provisions and returns the owning account, and a
+// second login for the same address resolves to that same account (a returning
+// user), with the identity confirmed.
+func TestEmailLoginFlow(t *testing.T) {
+	ctx := context.Background()
+	mailer := &capturingMailer{}
+	svc := account.NewEmailService(account.NewStore(testDB), mailer)
+	email := "login-" + uuid.NewString() + "@example.com"
+
+	accountID, err := svc.RequestLoginCode(ctx, email)
+	if err != nil {
+		t.Fatalf("request login code: %v", err)
+	}
+	code := sixDigit.FindString(mailer.lastBody)
+	if code == "" {
+		t.Fatalf("no code in mail body %q", mailer.lastBody)
+	}
+
+	acc, err := svc.LoginWithCode(ctx, email, code)
+	if err != nil {
+		t.Fatalf("login with code: %v", err)
+	}
+	if acc.ID != accountID {
+		t.Errorf("login account = %s, want %s", acc.ID, accountID)
+	}
+	if acc.IsGuest {
+		t.Error("an email account must be durable, not a guest")
+	}
+	if !identityConfirmed(t, account.KindEmail, email) {
+		t.Error("the email identity must be confirmed after login")
+	}
+
+	// A second login for the same email is the returning user: same account.
+	if _, err := svc.RequestLoginCode(ctx, email); err != nil {
+		t.Fatalf("second request: %v", err)
+	}
+	acc2, err := svc.LoginWithCode(ctx, email, sixDigit.FindString(mailer.lastBody))
+	if err != nil {
+		t.Fatalf("second login: %v", err)
+	}
+	if acc2.ID != accountID {
+		t.Errorf("returning login account = %s, want %s", acc2.ID, accountID)
+	}
+}