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/payments/store.go

@@ -14,13 +14,16 @@ import (
 // Store is the Postgres-backed query surface for the payments schema. It is the
 // only place in the backend that issues SQL against payments.* (an
 // import-boundary test enforces it), so the domain stays extractable into its
-// own database.
+// own database. It fronts the materialized balances/benefits tables with an
+// in-process write-through cache (see cache.go) so hot reads issue no query on
+// the steady-state path.
 type Store struct {
-	db *sql.DB
+	db    *sql.DB
+	cache *walletCache
 }
 
-// NewStore constructs a Store wrapping db.
-func NewStore(db *sql.DB) *Store { return &Store{db: db} }
+// NewStore constructs a Store wrapping db, with an empty read cache.
+func NewStore(db *sql.DB) *Store { return &Store{db: db, cache: newWalletCache()} }
 
 // Ping verifies the payments schema is reachable by reading the singleton config
 // row. It is the data-foundation health check; the wallet query surface arrives