docs: Add comprehensive Javadoc documentation to all Java files and update README

[staging-devin-ai-integration]

Summary

This PR adds comprehensive documentation to the springbootapp repository:

Javadoc Comments (116 files): Added class-level and method-level Javadoc documentation across all Java source and test files, covering:

• Core domain entities (Article, User, Comment, Tag)
• Repository interfaces and MyBatis implementations
• Application layer services (query services, command services, DTOs, pagination)
• REST API controllers and exception handlers
• GraphQL datafetchers and mutations
• Infrastructure layer (mappers, type handlers, read services)
• All test classes

README.md Enhancements: Significantly expanded the README with:

• Features overview
• Visual project structure diagram
• Requirements and dependencies table
• Configuration examples
• REST API usage examples with curl commands
• GraphQL query examples
• Docker instructions

Review & Testing Checklist for Human

• Verify Article.java regex line (line 149) - During editing, there was a character encoding issue with Unicode curly quotes in the regex pattern. The line was restored from master and re-documented. Please verify the regex [\\&|[\\uFE30-\\uFFA0]|\\'|\\"|\\s\\?\\,\\.]+ is intact and the slug generation tests pass.
• Spot-check Javadoc accuracy - Review a few Javadoc comments (especially in core domain classes like Article.java, User.java) to ensure descriptions accurately reflect the code behavior.
• Verify README API examples - Test a few of the curl commands in the README to ensure they work as documented.

Recommended test plan:

1. Run ./gradlew test to ensure all tests pass
2. Run ./gradlew bootRun and try a few API endpoints from the README examples
3. Verify the ArticleTest slug generation tests pass (these exercise the regex that had encoding issues)

Notes

Note: The ./gradlew spotlessJavaApply command fails due to a pre-existing Java version compatibility issue with Google Java Format (IllegalAccessError with jdk.compiler module). This is an environment configuration issue, not related to this PR's changes. CI may fail on this check.

Link to Devin run: https://samconfigtest.staging.devinenterprise.com/sessions/ddef19420a124b658201770508502350
Requested by: @samfert-codeium

[staging-devin-ai-integration]

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[sonarqubecloud]

Quality Gate failed

Failed conditions
2 Security Hotspots

See analysis details on SonarQube Cloud