feat(ads): VK post-move interstitial + retire deprecated hint_balance/paid_account

Interstitial video after a confirmed play or a hint, VK-only, offline
banner-only. The gate is client-mirrored: the backend puts the config
cooldowns (global 5m / vs_ai 30m / hint 1m) and a suppressed flag (the
no-ads / no_banner gate, same as the banner) on Profile.ads via adsFor;
the client self-gates on a per-kind last-shown time in localStorage, with
the VITE_ADS_STUB contour "ad fired" toast. Never fires after a pass,
exchange or resign. maybeShowInterstitial + vkShowInterstitial; the codec
and gateway transcode carry the ads block.

Also retires the deprecated accounts.hint_balance / paid_account domain
usage (expand-contract, code only — the columns stay for a later DROP so
image rollback stays DB-safe): drop the Account fields and their scan, the
dead account.SpendHint, account.GrantHints and the admin grant-hints
action; the in-game hint display now comes wholly from the payments hint
benefit. Banner eligibility and account merge no longer read the legacy
flags.

Tests: ads.test.ts (the client-mirrored gate), the codec ads round-trip,
the gateway ads transcode test, and a profile ads-config integration test
(cooldowns + suppressed under no-ads / no_banner). Docs: PAYMENTS §10 (+ru)
interstitial, the decision amends, backend README, the plan.

gateway/internal/transcode/transcode_ads_test.go

@@ -0,0 +1,65 @@
+package transcode_test
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"scrabble/gateway/internal/transcode"
+	fb "scrabble/pkg/fbs/scrabblefb"
+)
+
+// TestProfileGetEncodesAds verifies the gateway forwards the backend's interstitial-ad block
+// into the Profile payload: the three cooldowns and the suppressed flag the client-mirrored gate
+// reads. The encode is not exercised by the mock e2e (it bypasses the codec), so a dropped field
+// would only surface here.
+func TestProfileGetEncodesAds(t *testing.T) {
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodGet || r.URL.Path != "/api/v1/user/profile" {
+			t.Errorf("unexpected %s %q", r.Method, r.URL.Path)
+		}
+		_, _ = w.Write([]byte(`{"user_id":"u-1","display_name":"Kaya","preferred_language":"en",` +
+			`"ads":{"cooldown_global_s":300,"cooldown_vs_ai_s":1800,"cooldown_hint_s":60,"suppressed":true}}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, ok := reg.Lookup(transcode.MsgProfileGet)
+	if !ok {
+		t.Fatal("profile.get not registered")
+	}
+	payload, err := op.Handler(context.Background(), transcode.Request{UserID: "u-1"})
+	if err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+
+	ads := fb.GetRootAsProfile(payload, 0).Ads(nil)
+	if ads == nil {
+		t.Fatal("profile carries no ads block")
+	}
+	if ads.CooldownGlobalS() != 300 || ads.CooldownVsAiS() != 1800 || ads.CooldownHintS() != 60 {
+		t.Errorf("cooldowns = %d/%d/%d, want 300/1800/60", ads.CooldownGlobalS(), ads.CooldownVsAiS(), ads.CooldownHintS())
+	}
+	if !ads.Suppressed() {
+		t.Error("suppressed = false, want true")
+	}
+}
+
+// TestProfileGetNoAds verifies a profile without an ads block encodes none (the backend omits it
+// only on an internal failure; normally the block is always present).
+func TestProfileGetNoAds(t *testing.T) {
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte(`{"user_id":"u-1","display_name":"Kaya","preferred_language":"en"}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, _ := reg.Lookup(transcode.MsgProfileGet)
+	payload, err := op.Handler(context.Background(), transcode.Request{UserID: "u-1"})
+	if err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+	if p := fb.GetRootAsProfile(payload, 0); p.Ads(nil) != nil {
+		t.Error("profile without an ads block unexpectedly carries one")
+	}
+}