Add codebase visualization diagram (DayTrader 8)

[alexlang74]

Summary

Adds CODEBASE_DIAGRAM.md — a self-contained visualization of the entire DayTrader 8 codebase using ten Mermaid diagrams.

Diagrams Included

#	Diagram	Type
1	High-Level Layered Architecture — Browser → View → Controller → Service → Data layers with all components and data flows	graph TB
2	Package / Module Structure — All Java packages, their contents, and inter-package dependencies	graph LR
3	JPA Entity Relationships — Full ER diagram for Account, AccountProfile, Holding, Order, and Quote	erDiagram
4	TradeServices Strategy Pattern — Class diagram showing CDI-based runtime-mode injection (EJB3 / Direct / Session2Direct)	classDiagram
5	Servlet Request Flow — Sequence diagram tracing a login and a buy request end-to-end	sequenceDiagram
6	Asynchronous Order Processing — Flowchart covering all four order-processing modes (SYNCH, ASYNCH_CDI, ASYNCH_EJB_TIMER, ASYNCH_2PHASE) and the JMS/WebSocket push chain	flowchart
7	WebSocket Real-Time Market Update Architecture — JMS Topic → DTStreamer3MDB → CDI event → MarketSummaryWebSocket → browser clients	graph LR
8	JAX-RS REST API Map — All REST endpoints under /rest/quotes and /rest/broadcast	graph TD
9	Benchmark Primitive Servlets Taxonomy — Categorized mindmap of all ~73 Ping servlets (Servlet, JDBC, EJB, CDI, JMS, WebSocket, JAX-RS, HTTP/2, Bean Validation)	mindmap
10	Build & Deployment Pipeline — Maven build → WAR → Liberty/WildFly/Payara, Liberty features, and infrastructure dependencies	flowchart LR

No Code Changes

This PR only adds a new documentation file; no source code, configuration, or build files are modified.