R3: dashboards, docs and tracker bake-back

- Edge/UX dashboard: aggregate request-rate vs rejection-rate panel
  (gateway_rate_limited_total by class; no per-user labels).
- ARCHITECTURE §2/§11/§12/§13: body cap + explicit h2c sizing, the rate-limit
  observability pipeline and auto-flag policy, the admin-limiter note (and the
  caddy-path gap), the landing container topology; fixed the stale 120/min
  per-user figure.
- FUNCTIONAL (+_ru): the Throttled view and the reversible high-rate flag.
- gateway/backend/deploy READMEs, TESTING.md, root CLAUDE.md updated.
- PRERELEASE.md: R3 interview decisions + implementation refinements logged;
  tracker R3 -> done (this PR implements it; CI gates the merge).

backend/README.md

@@ -99,6 +99,14 @@ durable owner — then the durable account wins and a fresh session is minted fo
 The `accounts.paid_account`/`merged_into`/`merged_at` columns back this. This supersedes the
 Stage 8 `email.bind.*` edge surface (the `RequestCode`/`ConfirmCode` primitives stay).
 
+**R3** adds rate-limit observability: the gateway posts its periodic rejection
+summaries to `POST /api/v1/internal/ratelimit/report`; `internal/ratewatch` keeps a
+bounded in-memory episode window for the console's **Throttled** page and applies the
+conservative auto-flag — an account sustaining `BACKEND_HIGHRATE_FLAG_THRESHOLD`
+rejected calls within `BACKEND_HIGHRATE_FLAG_WINDOW` gets the soft, reversible
+`accounts.flagged_high_rate_at` marker (set-once; a badge in the user list and a
+**Clear** action on the user card; never an automatic ban).
+
 ## Package layout
 
 ```
@@ -121,6 +129,7 @@ internal/lobby/      # in-memory matchmaking pool (+ robot substitution) + frien
 internal/robot/      # human-like robot opponent: account pool, seed-derived strategy, move driver
 internal/adminconsole/ # server-rendered admin console (Go templates + embedded CSS, view models), served at /_gm
 internal/connector/  # backend gRPC client to the Telegram connector (operator broadcasts)
+internal/ratewatch/  # gateway rate-limit reports: episode window for the console + the high-rate auto-flag (R3)
 ```
 
 ## Configuration (environment)
@@ -153,6 +162,8 @@ internal/connector/  # backend gRPC client to the Telegram connector (operator b
 | `BACKEND_CONNECTOR_ADDR` | — | Telegram connector gRPC address for admin-console operator broadcasts. Empty disables broadcasts. |
 | `BACKEND_GUEST_REAP_INTERVAL` | `1h` | How often the abandoned-guest reaper sweeps. |
 | `BACKEND_GUEST_RETENTION` | `720h` | Account age past which a guest with no game seat is deleted. |
+| `BACKEND_HIGHRATE_FLAG_THRESHOLD` | `1000` | Gateway-reported rejected calls within the window past which an account is soft-flagged (R3). |
+| `BACKEND_HIGHRATE_FLAG_WINDOW` | `10m` | The rolling window those rejections accumulate over. |
 
 ## Run