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.

.dockerignore

@@ -0,0 +1,9 @@
+node_modules
+.next
+.git
+.env
+.env.local
+Dockerfile
+.dockerignore
+README.md
+node-*