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).

pkg/fbs/scrabble.fbs

@@ -848,3 +848,27 @@ table NotificationEvent {
   invitation:Invitation;
   state:StateView;
 }
+
+// WalletSegment is one chip balance in the wallet view: the funding source
+// ("vk"/"telegram"/"direct"), the chip count, and whether it is spendable in the current
+// execution context (false for a frozen VK-iOS balance or an untrusted platform).
+table WalletSegment {
+  source:string;
+  chips:int;
+  spendable:bool;
+}
+
+// Wallet is the user-facing wallet: the context-visible chip segments and the context-applicable
+// benefits — the no-ads term end as unix milliseconds (0 = no active term) or the perpetual
+// forever flag, and the available hint count.
+table Wallet {
+  segments:[WalletSegment];
+  ads_forever:bool;
+  ads_paid_until_ms:long;
+  hints:int;
+}
+
+// WalletBuyRequest buys a chip-priced value with chips: the product to buy.
+table WalletBuyRequest {
+  product_id:string;
+}