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/users.gohtml

@@ -17,7 +17,7 @@
 {{range .Items}}
 <tr>
 <td><a href="/_gm/users/{{.ID}}">{{.ID}}</a></td>
-<td>{{.DisplayName}}{{if .Guest}} <span class="pill">guest</span>{{end}}</td>
+<td>{{.DisplayName}}{{if .Guest}} <span class="pill">guest</span>{{end}}{{if .FlaggedHighRate}} <span class="pill">high-rate</span>{{end}}</td>
 <td>{{.Kind}}</td>
 <td>{{.Language}}</td>
 <td>{{.CreatedAt}}</td>