Adapt deployment for Dokploy

Replace the Gitea Actions SSH deploy with a Docker image
(serversideup/php fpm-nginx) and docker-compose setup. Store the
SQLite database inside the persistent storage volume, trust the
reverse proxy, and move admin credentials into config so they work
with cached configuration.

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

config/admin.php

@@ -0,0 +1,6 @@
+<?php
+
+return [
+    'user' => env('ADMIN_USER'),
+    'password' => env('ADMIN_PASS'),
+];