Add stage 1: authentication REST API

Slim 4 + SQLite todo-list API providing email/password registration,
login, and an authenticated GET /me endpoint. Stateless HS256 JWTs,
bcrypt password hashing, uniform JSON error envelope, and a SQL
migration runner. Includes PHPUnit feature tests and stage-1 docs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,6 @@
+/vendor/
+/storage/
+/.env
+composer.phar
+.phpunit.cache/
+.phpunit.result.cache