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.

docs/ARCHITECTURE.md

@@ -922,6 +922,28 @@ edits take effect on the next `profile.get` (open/reconnect/foreground), not mid
   list/detail, cleared by the operator, **never an automatic ban** and never a request
   gate. The Edge/UX dashboard graphs the aggregate request rate against the rejection
   rate by class.
+- **Temporary IP ban (prod-only):** with `GATEWAY_ABUSE_BAN_ENABLED` set, the gateway
+  enforces a fail2ban-style block keyed by client IP, fed by three signals: an IP that
+  sustains `GATEWAY_ABUSE_BAN_THRESHOLD` rate-limiter rejections within
+  `GATEWAY_ABUSE_BAN_WINDOW` (the IP-keyed public/email/admin classes — the user class
+  stays the soft-flag's concern, never the ban's), a **honeypot** decoy-path hit, and a
+  **honeytoken** (a planted bearer no real client holds, `GATEWAY_HONEYTOKEN`). A banned
+  IP is refused with **429** by an edge middleware (`abuseGuard`) before any work —
+  covering the Connect edge, the live stream and the static SPA/landing the per-op limiter
+  never gated. A rejection ban lasts `GATEWAY_ABUSE_BAN_DURATION`; a tripwire/honeytoken
+  hit is near-zero-false-positive and earns a longer fixed ban (1 h / 24 h). The ban is
+  **in-memory, single-instance and resets on restart**, like `ratewatch`; each ban
+  increments `gateway_abuse_banned_total` (`reason` = rejections/tripwire/honeytoken). The
+  decoy paths live only in the contour **caddy**, which tags them with `X-Scrabble-Honeypot`
+  (stripping any client-supplied value) and routes them to the gateway. It is **off by
+  default and only enabled in prod**: the ban keys by real client IP, which the shared-NAT
+  test contour does not expose (every client arrives as one address), so a ban there would
+  be self-inflicted — the honeypot/honeytoken still **log** in the contour, only the ban
+  *action* is gated. Operators see the active bans and lift them on the admin console's
+  **Throttled** page; the gateway syncs its active set to the backend every 30 s
+  (`POST /api/v1/internal/bans/sync`, network-trusted like the rejection report) and applies
+  the operator unbans the response returns, so a manual unban takes effect within the sync
+  interval.
 - Unauthenticated `GET /healthz` (liveness) and `GET /readyz` (readiness — the
   database answers a bounded ping and the session cache is warmed).
 - The backend serves a **second listener** — a gRPC server
@@ -932,7 +954,7 @@ edits take effect on the next `profile.get` (open/reconnect/foreground), not mid
 
 | Concern | Enforced by |
 | --- | --- |
-| Public rate limiting / anti-abuse | gateway (per-IP public/email/admin classes, per-user authenticated class; a request body cap of `GATEWAY_MAX_BODY_BYTES`; rejections are metered, summarised to the backend and surfaced in the admin console with a conservative reversible auto-flag — §11) |
+| Public rate limiting / anti-abuse | gateway (per-IP public/email/admin classes, per-user authenticated class; a request body cap of `GATEWAY_MAX_BODY_BYTES`; rejections are metered, summarised to the backend and surfaced in the admin console with a conservative reversible auto-flag — §11). In prod a **temporary IP ban** (`GATEWAY_ABUSE_BAN_ENABLED`) blocks an IP that sustains rejections or trips a **honeypot** decoy path / **honeytoken**, refused with 429 before any work; operators lift bans from the console. Off in the shared-NAT test contour, where the client IP is not real (§11) |
 | Telegram initData validation (bot-token HMAC) | the Telegram **validator**; the gateway delegates it over gRPC, so the bot token (the HMAC secret) lives only in the validator and the bot, never in the gateway |
 | Session minting; email-code / guest validation | gateway (with backend) |
 | Session → `user_id` resolution, `X-User-ID` injection | gateway |