refactor: clean repository, split profiles, add Flyway and harden upload flowFeat/security flyway upload tests

[YIMO691]

Summary

This PR completes a repository cleanup and engineering hardening pass for the blog system.

What changed

• cleaned repository structure

  • removed IDE/workspace artifacts from version control
  • removed runtime upload files from Git, kept uploads/.gitkeep
  • narrowed .gitignore rules to avoid blocking future SQL migrations

• split configuration by environment

  • application.yml now contains shared settings only
  • application-dev.yml for local development
  • application-prod.yml for environment-variable-based production config

• introduced Flyway

  • enabled migration-based schema management
  • aligned JPA usage with validation-oriented startup

• hardened upload flow

  • moved upload logic into UploadService
  • added file size, extension and MIME validation
  • normalized file paths and isolated runtime upload directory
  • returned proper image content types

• tightened admin authorization

  • restricted editor, upload and delete endpoints to admin
  • added controller-side admin guard for delete operation

• improved testability and CI

  • added H2-backed test profile for CI
  • isolated tests from local MySQL
  • updated workflow to run tests and package build with test profile

• synchronized documentation

  • README now matches profiles, Flyway, upload service and startup flow

Validation

• local mvn test passed
• local mvn -DskipTests package passed
• CI workflow runs with SPRING_PROFILES_ACTIVE=test

Notes

• image access remains public at /articles/image/**
• development still uses local settings from application-dev.yml

[YIMO691]

refactor: clean repo, split profiles, add Flyway, and harden upload flow