feat(accountdelete): anonymize-and-tombstone deletion core

New accountdelete package: AnonymizeAndTombstone journals every credential to the
retention log (reason=delete) then removes them (freeing email/vk/tg for reuse),
snapshots the real display name into deleted_display_name and scrubs the live one to
'Удалённый игрок', sets deleted_at, anonymizes the account's game-seat snapshots, and
drops its friendships/blocks/invitations/friend-codes/drafts/pending-codes — all in one
transaction. Chat, feedback and complaints are kept (the tombstone keeps their
no-cascade FKs valid). Session revocation + game forfeit are orchestrated a layer up.
Integration test covers journalling, tombstone/scrub and credential reuse.

backend/internal/inttest/delete_test.go

@@ -0,0 +1,92 @@
+//go:build integration
+
+package inttest
+
+import (
+	"context"
+	"database/sql"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+
+	"scrabble/backend/internal/account"
+	"scrabble/backend/internal/accountdelete"
+)
+
+// deletedFields reads a tombstoned account's retained real name and its deleted_at.
+func deletedFields(t *testing.T, accountID uuid.UUID) (name string, deletedAt sql.NullTime) {
+	t.Helper()
+	var dn sql.NullString
+	err := testDB.QueryRowContext(context.Background(),
+		"SELECT deleted_display_name, deleted_at FROM accounts WHERE account_id = $1", accountID).
+		Scan(&dn, &deletedAt)
+	if err != nil {
+		t.Fatalf("read deleted fields %s: %v", accountID, err)
+	}
+	return dn.String, deletedAt
+}
+
+// TestAnonymizeAndTombstone: deletion journals + frees the credentials, tombstones the
+// account, scrubs the live name while retaining the real one, and frees the creds for a
+// new account to reuse.
+func TestAnonymizeAndTombstone(t *testing.T) {
+	ctx := context.Background()
+	store := account.NewStore(testDB)
+	deleter := accountdelete.NewDeleter(testDB)
+
+	acc, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "ru", "handle", "Иван", "+03:00")
+	if err != nil {
+		t.Fatalf("provision: %v", err)
+	}
+	email := "del-" + uuid.NewString() + "@example.com"
+	if err := store.AttachIdentity(ctx, acc.ID, account.KindEmail, email, true); err != nil {
+		t.Fatalf("attach email: %v", err)
+	}
+	before, err := store.GetByID(ctx, acc.ID)
+	if err != nil {
+		t.Fatalf("load before: %v", err)
+	}
+
+	if err := deleter.AnonymizeAndTombstone(ctx, acc.ID); err != nil {
+		t.Fatalf("delete: %v", err)
+	}
+
+	// The live identities are gone.
+	if ids, err := store.Identities(ctx, acc.ID); err != nil || len(ids) != 0 {
+		t.Fatalf("identities after delete = %+v (err %v), want none", ids, err)
+	}
+	// Both credentials are journalled with reason=delete.
+	got := retainedRows(t, acc.ID)
+	if len(got) != 2 {
+		t.Fatalf("retained rows = %+v, want 2", got)
+	}
+	for _, r := range got {
+		if r.reason != "delete" {
+			t.Errorf("retained reason = %q, want delete", r.reason)
+		}
+	}
+	// The live name is scrubbed; the real one is retained; deleted_at is set.
+	after, err := store.GetByID(ctx, acc.ID)
+	if err != nil {
+		t.Fatalf("load after: %v", err)
+	}
+	if after.DisplayName != accountdelete.AnonymizedName {
+		t.Errorf("live display name = %q, want %q", after.DisplayName, accountdelete.AnonymizedName)
+	}
+	name, deletedAt := deletedFields(t, acc.ID)
+	if name != before.DisplayName {
+		t.Errorf("retained name = %q, want %q", name, before.DisplayName)
+	}
+	if !deletedAt.Valid || time.Since(deletedAt.Time) > time.Minute {
+		t.Errorf("deleted_at = %+v, want a recent timestamp", deletedAt)
+	}
+	// The credentials are free: a new account can claim the same email.
+	other, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Other", "")
+	if err != nil {
+		t.Fatalf("provision other: %v", err)
+	}
+	if err := store.AttachIdentity(ctx, other.ID, account.KindEmail, email, true); err != nil {
+		t.Fatalf("email should be free after deletion, got: %v", err)
+	}
+}