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.

backend/internal/account/account.go

@@ -43,9 +43,7 @@ var ErrNotFound = errors.New("account: not found")
 // local-time window (in TimeZone) during which the player is asleep, so the
 // turn-timeout sweeper does not auto-resign them inside it. (The robot opponent's
 // own sleep is anchored to its human opponent's timezone with a per-game drift,
-// computed in internal/robot, not from a robot account's away window.) HintBalance
-// is the player's wallet of purchasable hints, spent after a game's per-seat
-// allowance.
+// computed in internal/robot, not from a robot account's away window.)
 type Account struct {
 	ID                  uuid.UUID
 	DisplayName         string
@@ -53,7 +51,6 @@ type Account struct {
 	TimeZone            string
 	AwayStart           time.Time
 	AwayEnd             time.Time
-	HintBalance         int
 	BlockChat           bool
 	BlockFriendRequests bool
 	// VariantPreferences is the set of game variants (engine.Variant stable labels:
@@ -69,10 +66,6 @@ type Account struct {
 	// true (the default): the platform side-service skips out-of-app push for the
 	// account.
 	NotificationsInAppOnly bool
-	// PaidAccount marks a lifetime one-time-payment account. It is a service field
-	// (no purchase flow yet); an account linking & merge ORs it so a paid status is
-	// never lost when accounts are consolidated.
-	PaidAccount bool
 	// MergedInto is the primary account a retired (merged) secondary points at, or
 	// uuid.Nil for a live account. A tombstone keeps the row so the no-cascade
 	// foreign keys of a shared finished game stay valid.
@@ -563,52 +556,6 @@ func (s *Store) ProvisionGuest(ctx context.Context, browserTZ string) (Account,
 	return modelToAccount(row), nil
 }
 
-// SpendHint atomically decrements the account's hint wallet by one, returning
-// true when a hint was spent and false when the balance was already empty. The
-// guarded UPDATE keeps it safe under concurrent spends across the player's games.
-func (s *Store) SpendHint(ctx context.Context, id uuid.UUID) (bool, error) {
-	stmt := table.Accounts.
-		UPDATE(table.Accounts.HintBalance, table.Accounts.UpdatedAt).
-		SET(table.Accounts.HintBalance.SUB(postgres.Int(1)), postgres.TimestampzT(time.Now().UTC())).
-		WHERE(
-			table.Accounts.AccountID.EQ(postgres.UUID(id)).
-				AND(table.Accounts.HintBalance.GT(postgres.Int(0))),
-		)
-	res, err := stmt.ExecContext(ctx, s.db)
-	if err != nil {
-		return false, fmt.Errorf("account: spend hint %s: %w", id, err)
-	}
-	n, err := res.RowsAffected()
-	if err != nil {
-		return false, fmt.Errorf("account: spend hint rows %s: %w", id, err)
-	}
-	return n > 0, nil
-}
-
-// GrantHints adds n hints to the account's wallet and returns the new balance. n must be
-// positive: the additive update can only raise the balance, never lower it, so it enforces the
-// admin console's raise-only rule by construction and stays correct under a concurrent SpendHint.
-// It returns ErrNotFound when no account matches.
-func (s *Store) GrantHints(ctx context.Context, id uuid.UUID, n int) (int, error) {
-	if n <= 0 {
-		return 0, fmt.Errorf("account: grant hints %s: n must be positive, got %d", id, n)
-	}
-	stmt := table.Accounts.
-		UPDATE(table.Accounts.HintBalance, table.Accounts.UpdatedAt).
-		SET(table.Accounts.HintBalance.ADD(postgres.Int(int64(n))), postgres.TimestampzT(time.Now().UTC())).
-		WHERE(table.Accounts.AccountID.EQ(postgres.UUID(id))).
-		RETURNING(table.Accounts.HintBalance)
-
-	var row model.Accounts
-	if err := stmt.QueryContext(ctx, s.db, &row); err != nil {
-		if errors.Is(err, qrm.ErrNoRows) {
-			return 0, ErrNotFound
-		}
-		return 0, fmt.Errorf("account: grant hints %s: %w", id, err)
-	}
-	return int(row.HintBalance), nil
-}
-
 // FlagHighRate stamps the soft "suspected high-rate" marker with at, only when
 // the account is not already flagged — the first sustained episode wins, and a
 // re-flag after an operator clear starts a fresh timestamp. An infra marker, not
@@ -664,12 +611,10 @@ func modelToAccount(row model.Accounts) Account {
 		TimeZone:               row.TimeZone,
 		AwayStart:              row.AwayStart,
 		AwayEnd:                row.AwayEnd,
-		HintBalance:            int(row.HintBalance),
 		BlockChat:              row.BlockChat,
 		BlockFriendRequests:    row.BlockFriendRequests,
 		IsGuest:                row.IsGuest,
 		NotificationsInAppOnly: row.NotificationsInAppOnly,
-		PaidAccount:            row.PaidAccount,
 		MergedInto:             mergedInto,
 		FlaggedHighRateAt:      flaggedHighRateAt,
 		CreatedAt:              row.CreatedAt,