feat(payments): live payment-availability kill switch + per-account override

Let an operator disable purchases live from the admin — a whole rail/channel or
one account — and show the user a localized reason on their next attempt, so a
provider outage or a misconfig is explained instead of a silent dead button.

- rail kill switch (payments.rail_status, per rail direct:web / direct:android /
  vk / telegram): enabled + a per-language message, edited on the catalog page.
  Fail-open — a rail with no row stays enabled, so payments are never accidentally
  killed. The intake gate (CanPurchase in handleWalletOrder, before the order)
  returns payment_unavailable + the localized message, orthogonal to the security
  gates.
- per-account override (payments.account_payment_override, a row only for
  non-default): allow / deny / default, edited on the user card. "allow" bypasses
  ONLY the ops rail switch, never the security gates (trusted platform, the email
  anchor, the VK-iOS freeze, the min client version).
- wire: an additive ExecuteResponse.message envelope field (frozen-contract-safe);
  the gateway forwards a backend domain-error message; the client shows it on a
  payment_unavailable buy attempt.
- admin: rail toggles on the catalog page, the override control on the user card.
- tests: the pure gate (unit, TDD), the store + gate + override end-to-end
  (integration, migration 00016), the client (svelte-check / vitest).
- docs: PAYMENTS (+ru), the decisions log (D45/D46). Fiscalization stays
  cabinet-side (owner decision) — no itemized-receipt code.

Contour-safe: additive migration (two new tables, no wipe), the wire add is
additive, and fail-open so nothing is disabled until an operator acts.

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

@@ -72,6 +72,16 @@
 {{if or .Finance.Abuse .Finance.Loss}}<li><b>Refund risk</b> <span class="warn">{{if .Finance.Abuse}}abuse-flagged{{end}}{{if .Finance.Loss}} · loss {{.Finance.Loss}} chips{{end}}</span></li>{{end}}
 </ul>
 {{else}}<p class="note">no balances or benefits</p>{{end}}
+<h3>Payment override</h3>
+<form class="form" method="post" action="/_gm/users/{{.ID}}/purchase-override">
+<label>Purchases for this account
+<select name="override">
+<option value="default" {{if eq .PurchaseOverride "default"}}selected{{end}}>Default (follow the rail switch)</option>
+<option value="allow" {{if eq .PurchaseOverride "allow"}}selected{{end}}>Always allow (bypasses the rail switch only, not security)</option>
+<option value="deny" {{if eq .PurchaseOverride "deny"}}selected{{end}}>Always deny</option>
+</select></label>
+<div><button type="submit">Save</button></div>
+</form>
 <h3>Ledger</h3>
 {{$uid := .ID}}
 {{if .Finance.Ledger}}