Add Checkstyle linter to Java backend

[Copilot]

Adds Checkstyle-based code quality enforcement to the Spring Boot backend.

Implementation

• Maven plugin: Checkstyle 10.12.5 via maven-checkstyle-plugin 3.3.1, runs in validate phase
• Ruleset: Google Java Style with pragmatic adaptations (120 char lines, relaxed JavaDoc for non-public APIs)
• Build behavior: Reports violations as warnings, doesn't fail builds (failOnViolation=false)
• CI integration: Explicit linting step in GitHub Actions before test execution

Configuration highlights

Key checks enabled in backend/checkstyle.xml:

• No wildcard imports
• Naming conventions (package/class/method/variable)
• Line length and whitespace rules
• Common antipatterns (missing switch default, unused imports)
• Method complexity limits (150 lines, 7 parameters)

Usage

# Quick check
mvn checkstyle:check

# Generate HTML report
mvn checkstyle:checkstyle
# Report: target/site/checkstyle.html

Current codebase has 14 minor violations (star imports, 4 lines exceeding 120 chars).

Original prompt

Add a linter for the java project

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