Add jest preset for testcontainers Postgres + integration suite

[greatest0fallt1me]

Description

This is a backend issue for the GrantFox campaign. Provide a testcontainers preset for integration tests against an ephemeral Postgres.

Requirements and Context

• Preset config
• Used by integration suite
• Cleanup hooks
• Doc explains
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b task/testcontainers-pg

2. Implement changes
   • jest.preset.integration.js (new)
   • tests/integration/**
   • docs/integration-tests.md (new)
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

test: testcontainers Postgres for integration

Acceptance Criteria

• Preset works
• Suite runs
• Cleanup
• Doc shipped

Guidelines

• Minimum 90% test coverage on changed lines
• Input validation at the boundary; standardized error envelope
• Structured logging with correlation IDs
• Clear documentation and inline comments
• Timeframe: 96 hours

[adeboladee]

@adeboladee has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, Please I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @adeboladee to this issue.