docs(readme): add architecture mermaid diagram

Add an Architecture section with a Mermaid diagram of the full data path
(SPA/mobile clients -> layered Express backend -> MariaDB, and the uo-link
sidecar bridge to the ServUO shard) plus a Contents entry. Same diagram is
mirrored in the docs repo (docs/website/ARCHITECTURE.md).

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -25,6 +25,7 @@ The design reference is [BACKEND_DESIGN.md](https://gitea.whitlocktech.com/Runic
 
 ## Contents
 
+- [Architecture](#architecture)
 - [Tech stack](#tech-stack)
 - [Project structure](#project-structure)
 - [Prerequisites](#prerequisites)
@@ -44,6 +45,102 @@ The design reference is [BACKEND_DESIGN.md](https://gitea.whitlocktech.com/Runic
 
 ---
 
+## Architecture
+
+How the pieces fit together — the React SPA and native app talk to one Express backend
+(`router → controller → model → db`), which persists to MariaDB and bridges to the live
+game world only through the **uo-link** sidecar. The shard itself is never internet-facing.
+
+```mermaid
+flowchart TB
+    %% ---------- Clients ----------
+    subgraph clients["Clients"]
+        browser["Browser<br/>React + Vite SPA<br/>(public · wiki · admin)"]
+        mobile["Native mobile app<br/>(bearer tokens)"]
+    end
+
+    idp["SSO providers<br/>Google · Discord · custom OIDC"]
+    discord["Discord"]
+
+    %% ---------- Website (one repo) ----------
+    subgraph website["website/ &nbsp;— Node app (one repo)"]
+        direction TB
+
+        subgraph backend["server/ — Express backend"]
+            direction TB
+            mw["Middleware<br/>helmet · siteMode · noindex<br/>rateLimit · loginProtection · botScore · validate"]
+            router["Router /api/v1<br/>auth (web · mobile · sso) · public · admin"]
+            ctrl["Controllers"]
+            auth["Session layer (auth/)<br/>sessionService · JWT/cookie · bearer · SSO+PKCE"]
+            model["Models (.model + .db)<br/>raw parameterized SQL — no ORM"]
+            sse["SSE fan-out<br/>public stream (allowlist) · admin stream (sensitive)"]
+
+            subgraph shardutil["Shard integration (utils/)"]
+                ingest["shardIngest.js<br/>WS ingest dispatcher"]
+                restcli["uoLinkClient.js<br/>REST client (never throws)"]
+            end
+
+            secret["secretBox.js<br/>AES-256-GCM secrets at rest"]
+        end
+
+        bot["bot/<br/>Discord bot"]
+    end
+
+    db[("MariaDB<br/>users · posts · wiki · settings · activity<br/>mobileSessions · authProviders · userIdentities<br/>uoLinkConfig · shard_online/economy/houses/events")]
+
+    %% ---------- Shard side ----------
+    subgraph shardside["Game shard (never internet-facing)"]
+        direction TB
+        sidecar["uo-link sidecar<br/>(Rust) — the only bridge exposed"]
+        servuo["ServUO shard<br/>(C# plugin)"]
+    end
+
+    %% ---------- Edges ----------
+    browser <-->|"same-origin JSON + SSE (cookie)"| mw
+    mobile -->|"REST (bearer access/refresh)"| mw
+    browser -.->|"OAuth redirect + PKCE"| idp
+    auth -.->|"token exchange"| idp
+
+    mw --> router --> ctrl
+    ctrl --> auth
+    ctrl --> model
+    ctrl --> restcli
+    ctrl --> sse
+    auth --> model
+    model <--> db
+    auth -. reads/writes secrets .-> secret
+    restcli -. reads config/token .-> secret
+    ingest --> model
+    ingest --> sse
+    sse -->|"live events"| browser
+    bot -->|"messages"| discord
+    bot <--> db
+
+    restcli -->|"REST: /char /roster /economy /history · /link/confirm · /towncrier"| sidecar
+    sidecar -->|"WebSocket live event feed (bearer + X-UOLink-Version)"| ingest
+    servuo -->|"loopback TCP 127.0.0.1:7788<br/>newline-delimited JSON (shard dials out)"| sidecar
+
+    %% ---------- Styling ----------
+    classDef ext fill:#2d2233,stroke:#7a5c94,color:#e8dff0;
+    classDef store fill:#1f2d2a,stroke:#4c8c7d,color:#dff0ea;
+    classDef bridge fill:#2d2620,stroke:#94764c,color:#f0e6d8;
+    class idp,discord ext;
+    class db store;
+    class sidecar,servuo bridge;
+```
+
+- **One backend, layered.** Every request flows `middleware → router → controller → model → db`.
+  Web browsers authenticate with an httpOnly JWT cookie; the native app uses short-lived bearer
+  access tokens plus rotated refresh tokens; SSO (Google/Discord/OIDC) is link-only and PKCE-guarded.
+  All three surfaces produce the *same* session via the session layer.
+- **The shard is never reachable.** The ServUO shard *dials out* over loopback TCP to the uo-link
+  sidecar; only the sidecar is exposed, and only the backend talks to it. The REST client
+  (`uoLinkClient.js`) never throws, so the site degrades gracefully when the shard is down.
+- **Sensitive events stay private.** Ingested game events fan out to browsers over two SSE channels —
+  a public allowlist stream and an admin-only stream that adds staff audit / cheat / login events.
+
+---
+
 ## Tech stack
 
 | Layer | Tech |