Refactor to static markdown (remove Supabase backend) + self-host build

Posts now read from data/posts/*.md via gray-matter + remark; authors derived
from posts. Removes admin dashboard, comments, subscriptions, and all Supabase
usage. Adds Dockerfile (Next.js standalone) + Gitea Actions CI. output: standalone.

next.config.ts

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