docs: improve README with English product overview and user-focused onboarding

[GalaxySciTech]

Motivation

• Clarify the project purpose and design rationale by rewriting the README in clear English so new users and operators can quickly understand what the repo provides.
• Surface practical, business-oriented examples so readers can map features to real workloads (CEX, merchant payments, custodial services).
• Reduce onboarding friction by giving a concise quick-start and a 10-minute first-time user path that guides users to a working demo via Docker and Swagger.

Description

• Rewrote README.md to introduce the product, explain the single-service architecture, and highlight operational benefits in plain English.
• Added a "Product capabilities" section listing wallet lifecycle, deposit scanning, chain sync, sweep automation, fee scheduling, and admin operations.
• Added three typical use cases (Centralized Exchange, Payment/Merchant Platform, Custodial Service) and a concise quick-start with Docker Compose and Gradle run options.
• Reorganized runtime configuration, security recommendations, API entry points, database notes, and appended a prioritized list of README improvements (architecture diagram, E2E demo, API cookbook, environment matrix, troubleshooting, FAQ).

Testing

• Documentation-only change; no automated tests were required or executed.

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Codex Task