feat(payments): chip wallet, store-compliance gate and benefit application

Stand up the internal chip/benefit mechanic behind the narrow payments interface:
context-aware balances and benefits, an atomic chip spend, admin grants as
zero-price value sales, the one-directional store-compliance gate (VK/TG same-
origin only, web draws direct→vk→tg, VK-iOS frozen, untrusted fail-closed), and
per-origin hint and no-ads application with term stacking. Reads are served from
an in-process, account-keyed write-through cache (mirroring the suspension gate),
so hot paths issue no query to the payments schema.

Flip the online-game hint wallet and the ad-banner suppression from the deprecated
accounts.hint_balance / paid_account columns to the payments benefit (a hint
balance no longer suppresses the banner — only a no-ads benefit does), and fold
chip segments and benefits by origin on account merge, inside the merge tx. Add
the GET/POST /api/v1/user/wallet edge chain (REST → Connect → FlatBuffers) plus
its codec unit test; no wallet UI yet.

Bring the frozen owner decisions log into the repo at
docs/PAYMENTS_DECISIONS_ru.md (it was untracked under .vscode) and reference it
from PLAN.md; record the read-cache design and the present-sources interface in
PLAN.md and docs/PAYMENTS.md (+ RU mirror).

backend/internal/inttest/merge_test.go

@@ -58,14 +58,6 @@ func bestMoveCount(t *testing.T, id uuid.UUID) int {
 	return n
 }
 
-func setWallet(t *testing.T, id uuid.UUID, hints int, paid bool) {
-	t.Helper()
-	if _, err := testDB.ExecContext(context.Background(),
-		`UPDATE backend.accounts SET hint_balance=$2, paid_account=$3 WHERE account_id=$1`, id, hints, paid); err != nil {
-		t.Fatalf("set wallet: %v", err)
-	}
-}
-
 func bindEmailIdentity(t *testing.T, acc uuid.UUID, email string) {
 	t.Helper()
 	if _, err := testDB.ExecContext(context.Background(),
@@ -131,6 +123,7 @@ func TestAccountMergeCore(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
 	merger := accountmerge.NewMerger(testDB)
+	merger.SetPayments(newPaymentsService())
 
 	primary := provisionAccount(t)
 	secondary := provisionAccount(t)
@@ -140,8 +133,8 @@ func TestAccountMergeCore(t *testing.T) {
 	setStats(t, secondary, 3, 1, 2, 400, 80)
 	setStatsCounts(t, primary, 100, 5)
 	setStatsCounts(t, secondary, 50, 8)
-	setWallet(t, primary, 2, false)
-	setWallet(t, secondary, 5, true)
+	seedBenefit(t, primary, "telegram", 2, false)
+	seedBenefit(t, secondary, "telegram", 5, true) // hints sum, forever OR-s on merge
 	// Best moves: secondary's scrabble_en (80) beats primary's (50) and is kept; secondary's
 	// scrabble_ru (30) is new to primary and carried over.
 	setBestMove(t, primary, "scrabble_en", 50)
@@ -165,15 +158,13 @@ func TestAccountMergeCore(t *testing.T) {
 		t.Error("secondary stats row should be deleted after merge")
 	}
 
-	acc, err := store.GetByID(ctx, primary)
-	if err != nil {
-		t.Fatalf("get primary: %v", err)
+	// The payments wallet merged by origin: hints sum (2+5) and the forever flag OR-s; the
+	// deprecated accounts.hint_balance / paid_account columns are no longer touched by a merge.
+	if hints, forever := readBenefit(t, primary, "telegram"); hints != 7 || !forever {
+		t.Errorf("merged telegram benefit = hints %d forever %v, want 7/true", hints, forever)
 	}
-	if acc.HintBalance != 7 {
-		t.Errorf("hint balance = %d, want 7", acc.HintBalance)
-	}
-	if !acc.PaidAccount {
-		t.Error("paid_account should be true (ORed from secondary)")
+	if hints, _ := readBenefit(t, secondary, "telegram"); hints != 0 {
+		t.Errorf("secondary benefit should be cleared after merge, got hints %d", hints)
 	}
 
 	if owner, ok, _ := store.AccountIDByIdentity(ctx, account.KindEmail, email); !ok || owner != primary {