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>

phpcs.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0"?>
+<ruleset name="ScreamingFrog">
+    <description>PHP_CodeSniffer configuration</description>
+
+    <!-- Use PSR-12 coding standard -->
+    <rule ref="PSR12"/>
+
+    <!-- Paths to check -->
+    <file>/var/www/html</file>
+    <file>/var/www/tests</file>
+
+    <!-- Exclude vendor directory -->
+    <exclude-pattern>/var/www/html/vendor/*</exclude-pattern>
+    <exclude-pattern>*/vendor/*</exclude-pattern>
+
+    <!-- Show progress and colors -->
+    <arg name="colors"/>
+    <arg value="sp"/>
+</ruleset>