fix: remove hardcoded secrets and enable CSRF protection

[devin-ai-integration]

fix: remove hardcoded secrets and enable CSRF protection

Summary

This PR addresses three critical security vulnerabilities across the banking microservices application:

1. Replaced hardcoded OAuth2 client secrets with ${KEYCLOAK_CLIENT_SECRET} environment variable in API-Gateway and User-Service configurations
2. Enabled CSRF protection by removing .csrf().disable() from API-Gateway SecurityConfig (restores Spring Security's secure defaults)
3. Secured database credentials across all microservices (Account-Service, Sequence-Generator, Fund-Transfer, Transaction-Service) using environment variables with fallback defaults (User-Service already had this pattern)
4. Added comprehensive documentation in README.md for all required environment variables

All changes maintain backward compatibility through environment variable defaults while eliminating hardcoded secrets from version control.

Review & Testing Checklist for Human

• Test CSRF compatibility: Verify existing API consumers (frontend, Postman collections) still work after enabling CSRF protection. Check for 403 errors on POST/PUT/DELETE operations
• Verify authentication flows: Test OAuth2/JWT authentication still works with KEYCLOAK_CLIENT_SECRET environment variable instead of hardcoded values
• Test database connectivity: Ensure all microservices can connect to MySQL databases with environment variable configuration
• Environment setup: Confirm all required environment variables are documented and can be properly configured in deployment environments

Notes

• CSRF protection is now enabled using Spring WebFlux's default WebSessionServerCsrfTokenRepository - this may require frontend applications to handle CSRF tokens
• Environment variables use Spring's ${VAR:default} syntax, so services will still start with default values if variables aren't set
• Expanded scope to secure all microservices with database connections, not just the three originally mentioned

Requested by: Sam Fertig (@samfert-codeium)
Link to Devin run: https://app.devin.ai/sessions/6d1c848f9df141ee88aee61e8e9e5316

[devin-ai-integration]

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