Add architecture diagram to README (#19)

* Add Mermaid architecture diagram to README

Show the request flow from HTTP clients through middleware layers
(CORS, logging, error handling, rate limiting, auth) into the
router endpoints, the shared MT5 client singleton, and down to the
MetaTrader 5 terminal via pdmt5.

https://claude.ai/code/session_01YNwrPB3iN3JWpkVgM5XwVQ

* Compact FastAPI Application subgraph in architecture diagram

Collapse the middleware chain and seven routers into single summary
nodes to reduce diagram verbosity while keeping the key layers visible.

https://claude.ai/code/session_01YNwrPB3iN3JWpkVgM5XwVQ

* docs: clarify architecture diagram

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

README.md

@@ -14,6 +14,32 @@ The API server must run on Windows. The `MetaTrader5` Python package used by
 machine with a logged-in MetaTrader 5 terminal. HTTP clients can connect from
 any operating system.
 
+## Architecture
+
+```mermaid
+graph TB
+    Client["HTTP Client<br/>(Any OS)"]
+
+    subgraph "Windows Host"
+        subgraph "FastAPI Application"
+            Middleware["Middleware Stack<br/>CORS · Logging · Error Handler · Rate Limiter"]
+            Routers["Routers<br/>health · symbols · market · account · history · calc · trading"]
+            Auth["API Key Security Dependency<br/>Security(api_key_header) · verify_api_key"]
+            Deps["FastAPI Dependencies<br/>MT5 Client Singleton · Format Negotiation"]
+            Formatters["Response Formatters<br/>JSON · Parquet"]
+            Middleware --> Routers --> Deps --> Formatters
+            Auth -.-> Routers
+            Formatters --> Middleware
+        end
+
+        Deps --> pdmt5["pdmt5<br/>MT5 Client Library"]
+        pdmt5 --> MT5["MetaTrader 5<br/>Terminal"]
+    end
+
+    Client -- "HTTP/REST" --> Middleware
+    Middleware -- "JSON / Parquet" --> Client
+```
+
 ## Features
 
 - REST endpoints for symbols, market data, account info, orders, history,