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>

phpunit.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+         cacheDirectory=".phpunit.cache">
+    <testsuites>
+        <testsuite name="Feature">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+    <php>
+        <env name="APP_DEBUG" value="true"/>
+        <env name="JWT_SECRET" value="test-secret-do-not-use-in-production"/>
+    </php>
+</phpunit>