Add static analysis: Checkstyle/SpotBugs (backend) + ESLint/Prettier (frontend)

[pcastelo]

Problem

No static analysis tools configured for either backend or frontend. Issues like unused imports, dead code, style violations, and complexity go undetected.

Acceptance criteria

• Backend: Add Checkstyle or SpotBugs Maven plugin
  • Configure with reasonable ruleset (Google Java Style or similar)
  • Fail build on critical violations
• Frontend: Add ESLint with @angular-eslint
  • Enable recommended rules + strict TypeScript rules
  • Configure no-any, no-unused-vars, prefer-const
• Add Prettier for consistent formatting (frontend)
• Add linting step to pr-check.yml
• Fix all existing violations or establish baseline with explicit suppressions