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).

gateway/internal/backendclient/api.go

@@ -360,6 +360,42 @@ func (c *Client) Profile(ctx context.Context, userID string) (ProfileResp, error
 	return out, err
 }
 
+// WalletSegmentResp is one chip balance in the wallet: the funding source, the chip count, and
+// whether it is spendable in the caller's current context.
+type WalletSegmentResp struct {
+	Source    string `json:"source"`
+	Chips     int    `json:"chips"`
+	Spendable bool   `json:"spendable"`
+}
+
+// WalletResp is the caller's wallet: the context-visible chip segments and the context-applicable
+// benefits (no-ads term end as unix millis / forever flag, and the available hints).
+type WalletResp struct {
+	Segments     []WalletSegmentResp `json:"segments"`
+	AdsForever   bool                `json:"ads_forever"`
+	AdsPaidUntil int64               `json:"ads_paid_until_ms"`
+	Hints        int                 `json:"hints"`
+}
+
+// walletBuyBody is the chip-spend request body.
+type walletBuyBody struct {
+	ProductID string `json:"product_id"`
+}
+
+// Wallet fetches the caller's wallet in their current execution context.
+func (c *Client) Wallet(ctx context.Context, userID string) (WalletResp, error) {
+	var out WalletResp
+	err := c.do(ctx, http.MethodGet, "/api/v1/user/wallet", userID, "", nil, &out)
+	return out, err
+}
+
+// WalletBuy spends chips on a chip-priced value and returns the updated wallet.
+func (c *Client) WalletBuy(ctx context.Context, userID, productID string) (WalletResp, error) {
+	var out WalletResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/wallet/buy", userID, "", walletBuyBody{ProductID: productID}, &out)
+	return out, err
+}
+
 // BlockStatusResp is the caller's current manual-block state. Until is an RFC3339 UTC instant for
 // a temporary block, empty for a permanent one or when not blocked; Reason is resolved to the
 // account's language, empty when none was cited.