docs: add system architecture overview with diagrams

[Delightech28]

docs: add system architecture overview with diagrams

Description

Add a comprehensive system architecture overview that shows how Soroban contracts, backend API, event indexer, PostgreSQL database, and Next.js frontend fit together.

Changes

• wiki/architecture.md (new) – Complete architecture guide with:

  • High-level component diagram connecting all layers
  • Request flow diagram (user action → contract)
  • Event processing flow (contract → indexer → database)
  • Dashboard load flow (frontend → cache/database)
  • Trust boundaries and security model
  • External dependencies (Stellar RPC, PostgreSQL, Redis, Sentry)
  • Cross-links to related docs

• wiki/README.md (updated) – Added architecture.md as first item in Contents

Acceptance Criteria

• Component diagram of contracts, backend, indexer, DB, and frontend
• Request and event data flows documented
• Links to contract reference, API reference, and indexer-sync-flow
• Trust boundaries identified
• External dependencies listed

Closes #1