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

package.json

@@ -24,6 +24,7 @@
         "@tailwindcss/typography": "0.5.16",
         "@xterm/addon-fit": "0.10.0",
         "@xterm/xterm": "5.5.0",
-        "ioredis": "5.6.1"
+        "ioredis": "5.6.1",
+        "playwright": "^1.58.2"
     }
-}
+}