R3: backend rate-limit observability — ratewatch, auto-flag, admin throttled view

- accounts.flagged_high_rate_at baked into the R1 baseline (no prod data; the
  contour schema is wiped after merge); jet regenerated — the regen also picks
  up the previously missing game_drafts/game_hidden models.
- account.Store: FlagHighRate (set-once), ClearHighRateFlag, the flag in
  GetByID/ListUsers and a ListFlaggedHighRate review queue.
- New internal/ratewatch: ingests the gateway rejection reports, keeps a
  bounded in-memory episode window for the console and applies the
  conservative auto-flag (1000 rejected / 10 min, BACKEND_HIGHRATE_FLAG_*).
- POST /api/v1/internal/ratelimit/report (network-trusted, like
  sessions/resolve).
- Admin console: Throttled page (episodes + flagged accounts), a high-rate
  badge in the user list, the marker + operator clear action on the user card.
- Tests: ratewatch unit suite, report-route handler test, renderer cases,
  integration coverage for the store round-trip and the console flow.

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

@@ -0,0 +1,39 @@
+{{define "content" -}}
+<h1>Throttled</h1>
+{{with .Data}}
+<p class="note">Rate-limiter rejections reported periodically by the gateway. The episode
+list is in-memory and resets on a backend restart. An account sustaining
+{{.FlagThreshold}}+ rejected calls within {{.FlagWindow}} is soft-flagged for review
+below — never banned automatically; clear the flag on the user card.</p>
+<section class="panel"><h2>Recent episodes</h2>
+<table class="list">
+<thead><tr><th>Class</th><th>Key</th><th class="num">Rejected</th><th>First seen</th><th>Last seen</th></tr></thead>
+<tbody>
+{{range .Episodes}}
+<tr>
+<td>{{.Class}}</td>
+<td>{{if .UserID}}<a href="/_gm/users/{{.UserID}}">{{.Key}}</a>{{else}}<code>{{.Key}}</code>{{end}}</td>
+<td class="num">{{.Rejected}}</td>
+<td>{{.FirstSeen}}</td>
+<td>{{.LastSeen}}</td>
+</tr>
+{{else}}
+<tr><td colspan="5"><span class="note">nothing throttled recently</span></td></tr>
+{{end}}
+</tbody>
+</table>
+</section>
+<section class="panel"><h2>Flagged accounts</h2>
+<table class="list">
+<thead><tr><th>Account</th><th>Display name</th><th>Flagged</th></tr></thead>
+<tbody>
+{{range .Flagged}}
+<tr><td><a href="/_gm/users/{{.ID}}">{{.ID}}</a></td><td>{{.DisplayName}}</td><td>{{.FlaggedAt}}</td></tr>
+{{else}}
+<tr><td colspan="3"><span class="note">no flagged accounts</span></td></tr>
+{{end}}
+</tbody>
+</table>
+</section>
+{{end}}
+{{- end}}