docs: comprehensive technical assessment — knowledge base, gap analysis, and remediation roadmap

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Summary

Comprehensive technical assessment of the Java 21 / Spring Boot 3.2.4 internet banking microservices application. This PR adds three documentation deliverables under docs/:

1. KNOWLEDGE_BASE.md — Full application knowledge base covering:

   • Architecture overview (6 services, communication patterns, infrastructure)
   • Data model documentation (entities, relationships, key fields per service)
   • API surface map (all endpoints across all services with methods and request/response shapes)
   • Business logic inventory (fund transfer rules, payment processing, user management)
   • Integration points (Keycloak, Zipkin, MySQL, Spring Cloud Config)
   • Build and deployment pipeline summary (Docker Compose, Gradle)

2. GAP_ANALYSIS.md — Engineering standards gap analysis identifying 44 gaps across 7 categories:

   • Code organization (5 gaps) — duplicated code, no shared library, inconsistent packages
   • Error handling (6 gaps) — stack trace leakage, wrong HTTP status codes, inconsistent formats
   • Testing (6 gaps) — tests only in core-banking-service, no integration/contract tests
   • Security (10 gaps) — hardcoded secrets, no input validation, password logging
   • API design (8 gaps) — raw ResponseEntity types, missing pagination metadata, wrong Swagger dependency
   • Observability (7 gaps) — no structured logging, no Prometheus metrics, sensitive data in logs
   • Resilience (9 gaps) — no circuit breakers, no timeouts, balance calculation bug, non-atomic transactions

   Each gap rated by severity (Critical/High/Medium/Low) and remediation effort (Small/Medium/Large).

3. REMEDIATION_ROADMAP.md — Phased plan with 24 remediation items and ready-to-use Devin prompts:

   • Phase 1 (Quick wins, 1–2 weeks): Bug fixes, security hardening, input validation, error codes
   • Phase 2 (Important, 3–6 weeks): Circuit breakers, shared library, test coverage, observability
   • Phase 3 (Polish, 6–12 weeks): Contract tests, CI/CD pipeline, filtering, notification service

Review & Testing Checklist for Human

• Review docs/KNOWLEDGE_BASE.md for accuracy — verify the architecture diagram, API endpoint list, and data model tables match the actual codebase
• Review docs/GAP_ANALYSIS.md — confirm severity ratings align with your team's priorities; check that the balance calculation bug (RE-8) is accurate
• Review docs/REMEDIATION_ROADMAP.md — validate that Devin prompts are actionable and phase prioritization makes sense for your roadmap
• Verify no existing documentation files were modified or overwritten

Notes

• The balance calculation bug (RE-8) in TransactionService.java is a confirmed defect: availableBalance is double-deducted because actualBalance.subtract(amount) is called on an already-reduced balance.
• RabbitMQ is mentioned in the README but is not yet integrated in the codebase; the notification service is pending development.
• The Swagger dependency (springdoc-openapi-starter-webflux-ui) is incorrect for Spring MVC services; documented in AD-5.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/0480d5a6e76a482e8106534bd9c5eb56
Requested by: @DhrovS

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.