Stage 10: admin console & dictionary ops (complaint review, hot-reload, broadcasts)

Server-rendered admin console in the backend at /_gm (internal/adminconsole),
fronted on the gateway's public listener by Basic-Auth + a verbatim reverse proxy
(mounted on the edge mux below the h2c wrap). A same-origin check guards its POSTs;
no operator identity is tracked. This supersedes the Stage 6 gateway-fronts-
/api/v1/admin model: GATEWAY_ADMIN_ADDR and the backend /api/v1/admin ping are
dropped and gateway/internal/admin is repurposed to the verbatim proxy.

- Complaints: migration 00008 (+ jetgen) adds disposition/resolution_note/
  resolved_at/applied_in_version + the deferred status CHECK; resolution feeds a
  query-derived pending dictionary-change pipeline (marked applied after a reload).
- Dictionary hot-reload: per-version subdir BACKEND_DICT_DIR/<version>/ via the new
  Registry.LoadAvailable; engine.OpenWithVersions restores resident versions on
  restart. Partially addresses TODO-2.
- Broadcasts: a backend Telegram-connector client (internal/connector,
  BACKEND_CONNECTOR_ADDR) for SendToUser / SendToGameChannel (discharges the Stage 9
  forward-note).
- Admin reads: account.ListAccounts/CountAccounts/Identities and
  game.ListGames/CountGames/GameByID/ListComplaints/GetComplaint/CountComplaints/
  ResolveComplaint/DictionaryChanges/MarkChangesApplied.
- Tests: adminconsole render, engine reload, same-origin guard, gateway verbatim
  proxy + h2c console mount, inttest complaint pipeline + list/count + /_gm console.
- Docs: PLAN (Stage 10 done + refinements + TODO-2), ARCHITECTURE §1/§5/§6/§12/§13,
  FUNCTIONAL (+_ru), TESTING, backend/gateway READMEs.

gateway/internal/admin/admin.go

@@ -1,8 +1,11 @@
-// Package admin is the gateway's admin surface: HTTP Basic-Auth in front of a
-// reverse proxy to the backend admin API (docs/ARCHITECTURE.md §12). The gateway
-// validates the operator credential and forwards authenticated requests to
-// backend /api/v1/admin/*; the backend trusts the gateway on this segment. The
-// admin API itself is filled in Stage 10.
+// Package admin is the gateway's admin edge: HTTP Basic-Auth in front of a reverse
+// proxy that forwards the operator's browser to the backend's server-rendered admin
+// console under /_gm. The proxy is mounted at /_gm/ on the gateway's public listener
+// (below the h2c wrap, see internal/connectsrv) and forwards verbatim — an inbound
+// /_gm/<rest> reaches <backendURL>/_gm/<rest>, preserving the inbound Host so the
+// backend's same-origin check sees the public origin. The backend trusts the gateway
+// on this segment and adds the console's same-origin CSRF guard
+// (docs/ARCHITECTURE.md §12).
 package admin
 
 import (
@@ -11,17 +14,14 @@ import (
 	"net/http"
 	"net/http/httputil"
 	"net/url"
-	"strings"
 
 	"go.uber.org/zap"
 )
 
-// backendAdminPrefix is where the backend mounts its admin API.
-const backendAdminPrefix = "/api/v1/admin"
-
 // NewProxy returns a handler that checks Basic-Auth against user/password and
-// reverse-proxies the request to the backend admin API, mapping an inbound
-// /admin/<rest> path to <backendURL>/api/v1/admin/<rest>.
+// reverse-proxies the request verbatim to the backend: the inbound path is
+// preserved, so /_gm/<rest> reaches <backendURL>/_gm/<rest>. It is mounted at /_gm/
+// on the gateway's public listener.
 func NewProxy(backendURL, user, password string, log *zap.Logger) (http.Handler, error) {
 	target, err := url.Parse(backendURL)
 	if err != nil {
@@ -32,10 +32,8 @@ func NewProxy(backendURL, user, password string, log *zap.Logger) (http.Handler,
 	}
 	proxy := &httputil.ReverseProxy{
 		Rewrite: func(pr *httputil.ProxyRequest) {
-			pr.SetURL(target)
-			rel := strings.TrimPrefix(pr.In.URL.Path, "/admin")
-			pr.Out.URL.Path = backendAdminPrefix + rel
-			pr.Out.Host = pr.In.Host
+			pr.SetURL(target)        // backend scheme+host; the inbound /_gm path is preserved
+			pr.Out.Host = pr.In.Host // keep the public Host for the backend same-origin check
 		},
 		ErrorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
 			log.Warn("admin proxy upstream error", zap.String("path", r.URL.Path), zap.Error(err))