test: add Pest browser testing with SQLite :memory: schema

Set up end-to-end browser testing using Pest Browser Plugin + Playwright.
New v4 test suite uses SQLite :memory: database with pre-generated schema dump
(database/schema/testing-schema.sql) instead of running migrations, enabling
faster test startup.

- Add pestphp/pest-plugin-browser dependency
- Create GenerateTestingSchema command to export PostgreSQL schema to SQLite
- Add .env.testing configuration for isolated test environment
- Implement v4 test directory structure (Feature, Browser, Unit tests)
- Update Pest skill documentation with browser testing patterns, API reference,
  debugging techniques, and common pitfalls
- Configure phpunit.xml and tests/Pest.php for v4 suite
- Update package.json and docker-compose.dev.yml for testing dependencies

composer.json

@@ -69,6 +69,7 @@
         "mockery/mockery": "^1.6.12",
         "nunomaduro/collision": "^8.8.3",
         "pestphp/pest": "^4.3.2",
+        "pestphp/pest-plugin-browser": "^4.2",
         "phpstan/phpstan": "^2.1.38",
         "rector/rector": "^2.3.5",
         "serversideup/spin": "^3.1.1",