feat(payments): per-channel Robokassa shops on the direct rail

Split the single Robokassa direct rail into one merchant shop per channel
(web / android; ios later), chosen by the trusted X-Platform subtype, while
every shop still credits the one `direct` wallet — merchant-account separation
for accounting and receipts, not a new wallet.

- config: a channel-keyed shops registry (web seeds from the legacy vars or
  _WEB_*, android from _ANDROID_*); an empty shop leaves the rail dormant;
  per-shop validation.
- intake: the order picks its shop by subtype (unknown falls back to web); the
  per-shop Result callback is verified by that shop's own Password2 at
  /pay/robokassa/result/<channel> (the gateway extracts the channel; Caddy's
  /pay/* glob already forwards it — no Caddyfile change).
- persistence: an additive `shop` column on the order (migration 00015),
  recorded from the payment context, surfaced per entry in the admin report.
- standalone apps sign in by email only, so a direct purchase keeps its email
  anchor.
- docs: PAYMENTS (+ru) topology, deploy env vars + compose mapping, the
  decisions log (D41 revised for the ИП / 54-ФЗ move; D42-D44), the plan.

Contour-safe: dormant until shops are configured; the migration is additive
(no wipe); no client wire change. Fiscalization (Receipt/Email) and the gateway
`direct/android` subtype follow when the ИП / RuStore are live.

backend/internal/adminconsole/templates/pages/user_detail.gohtml

@@ -76,10 +76,10 @@
 {{$uid := .ID}}
 {{if .Finance.Ledger}}
 <table class="list">
-<thead><tr><th>Time</th><th>Kind</th><th>Source</th><th>Origin</th><th>Chips</th><th>Order</th><th>Provider</th><th>Detail</th><th></th></tr></thead>
+<thead><tr><th>Time</th><th>Kind</th><th>Source</th><th>Origin</th><th>Chips</th><th>Order</th><th>Provider</th><th>Shop</th><th>Detail</th><th></th></tr></thead>
 <tbody>
 {{range .Finance.Ledger}}
-<tr><td>{{.At}}</td><td>{{.Kind}}</td><td>{{.Source}}</td><td>{{.Origin}}</td><td>{{.ChipsDelta}}</td><td>{{if .Order}}<code>{{.Order}}</code>{{end}}</td><td>{{.Provider}}</td><td>{{if .Snapshot}}<code>{{.Snapshot}}</code>{{end}}</td>
+<tr><td>{{.At}}</td><td>{{.Kind}}</td><td>{{.Source}}</td><td>{{.Origin}}</td><td>{{.ChipsDelta}}</td><td>{{if .Order}}<code>{{.Order}}</code>{{end}}</td><td>{{.Provider}}</td><td>{{.Shop}}</td><td>{{if .Snapshot}}<code>{{.Snapshot}}</code>{{end}}</td>
 <td>{{if and (eq .Kind "fund") .Order}}<form class="form" method="post" action="/_gm/users/{{$uid}}/refund" onsubmit="return confirm('Refund this order in full? Record the money refund on the rail first; this revokes the chips (floored at 0).')"><input type="hidden" name="order_id" value="{{.Order}}"><button type="submit">Refund</button></form>{{end}}</td></tr>
 {{end}}
 </tbody>

backend/internal/adminconsole/views.go

@@ -232,8 +232,8 @@ type BenefitRow struct {
 }
 
 // LedgerRow is one append-only ledger entry: its kind, funding source / benefit origin, signed chip
-// delta, the product / order / provider it references (empty when none), the raw snapshot JSON and
-// the pre-formatted time.
+// delta, the product / order / provider / direct-rail shop it references (empty when none), the raw
+// snapshot JSON and the pre-formatted time.
 type LedgerRow struct {
 	Kind       string
 	Source     string
@@ -242,6 +242,7 @@ type LedgerRow struct {
 	Product    string
 	Order      string
 	Provider   string
+	Shop       string
 	Snapshot   string
 	At         string
 }