Add Docker build and Gitea Actions CI for deployment

Multi-stage Alpine Dockerfile producing a standalone Next.js image
with the better-sqlite3 native addon included. Gitea workflow builds
on push to main and on v* tags, pushing to the Gitea registry using
the REGISTRY_URL variable and REGISTRY_TOKEN secret. Compose file
mounts ./data so alerts.db and settings.json persist across restarts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

next.config.ts

@@ -1,7 +1,7 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  output: "standalone",
 };
 
 export default nextConfig;