Add comprehensive quality tooling and fix code style issues

Quality Tools Added:
- PHPStan (Level 8) for static analysis
- PHP_CodeSniffer (PSR-12) for code style
- Updated PHPUnit test suite with type safety

Code Improvements:
- Fixed all PHPStan Level 8 errors (13 issues)
- Auto-fixed 25 PSR-12 code style violations
- Added proper type hints for arrays and method parameters
- Fixed PDOStatement|false handling in api.php and tests
- Improved null-safety for parse_url() calls

Configuration:
- phpstan.neon: Level 8, analyzes src/ and tests/
- phpcs.xml: PSR-12 standard, excludes vendor/
- docker-compose.yml: Mount config files for tooling
- composer.json: Add phpstan, phpcs, phpcbf scripts

Documentation:
- Updated README.md with testing and quality sections
- Updated AGENTS.md with quality gates and workflows
- Added pre-commit checklist for developers

All tests pass (9/9), PHPStan clean (0 errors), PHPCS compliant (1 warning)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docker-compose.yml

@@ -13,6 +13,8 @@ services:
       - ./tests:/var/www/tests
       - ./composer.json:/var/www/composer.json
       - ./composer.lock:/var/www/composer.lock
+      - ./phpstan.neon:/var/www/phpstan.neon
+      - ./phpcs.xml:/var/www/phpcs.xml
       - ./config/nginx/default.conf:/etc/nginx/conf.d/default.conf
     depends_on:
       - mariadb