feat(gateway): temporary IP ban (fail2ban) fed by rejections + honeypot/honeytoken

Add a prod-only, in-memory IP ban enforced at the edge, fed by three signals:
sustained rate-limiter rejections (the IP-keyed public/email/admin classes — the
user class stays the backend soft-flag's concern), a honeypot decoy-path hit (the
contour caddy tags decoys with X-Scrabble-Honeypot and routes them to the gateway),
and a honeytoken (a planted bearer, GATEWAY_HONEYTOKEN). A banned IP is refused with
429 by the abuseGuard middleware before any work — covering the Connect edge, the
live stream and the static SPA/landing the per-op limiter never gated.

The ban is off by default: it keys by the real client IP the shared-NAT test contour
does not expose, so a ban there would be self-inflicted; detection still logs in the
contour, only the ban action is gated (GATEWAY_ABUSE_BAN_ENABLED). Rejection bans last
GATEWAY_ABUSE_BAN_DURATION; tripwire/honeytoken hits are near-zero-false-positive and
earn longer fixed bans. Each ban increments gateway_abuse_banned_total{reason}.

Operators see and lift active bans on the admin console's Throttled page; the gateway
syncs its active set to the backend every 30s (POST /api/v1/internal/bans/sync,
backend/internal/banview) and applies the operator unbans the response returns.

PRERELEASE phase AG. Docs baked into ARCHITECTURE / FUNCTIONAL (+ru) / both READMEs.

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

@@ -5,6 +5,26 @@
 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>Active IP bans</h2>
+<p class="note">Temporary IP bans the gateway is currently enforcing (in-memory, prod-only;
+reset on a gateway restart). Unban applies on the gateway's next sync.</p>
+<table class="list">
+<thead><tr><th>IP</th><th>Reason</th><th>Since</th><th>Expires</th><th></th></tr></thead>
+<tbody>
+{{range .Bans}}
+<tr>
+<td><code>{{.IP}}</code></td>
+<td>{{.Reason}}</td>
+<td>{{.Since}}</td>
+<td>{{.Expires}}</td>
+<td><form class="form" method="post" action="/_gm/bans/unban"><input type="hidden" name="ip" value="{{.IP}}"><button type="submit">Unban</button></form></td>
+</tr>
+{{else}}
+<tr><td colspan="5"><span class="note">no active bans</span></td></tr>
+{{end}}
+</tbody>
+</table>
+</section>
 <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>