feat(docker): add FrankenPHP Caddy configuration

Add production and dev Caddyfiles for FrankenPHP:
- Caddyfile: HTTP-only on :8080, auto_https off, worker mode
- Caddyfile.dev: same but without worker directive so Xdebug
  step-debugging remains functional; Caddy debug logging enabled

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker/frankenphp/Caddyfile

@@ -0,0 +1,13 @@
+{
+    admin off
+    auto_https off
+
+    frankenphp {
+        worker /app/public/worker.php
+    }
+}
+
+:8080 {
+    root * /app/public
+    php_server
+}

docker/frankenphp/Caddyfile.dev

@@ -0,0 +1,12 @@
+{
+    admin off
+    auto_https off
+    debug
+
+    frankenphp
+}
+
+:8080 {
+    root * /app/public
+    php_server
+}