Add Dockerfile + docker-compose for Coolify deployment

Multi-stage build: node builds the SPA, dotnet publishes the API, the
runtime image serves the SPA from wwwroot/ and exposes /health. Frontend
Supabase URL + publishable key are baked in at build time; DB conn string
and Supabase JWKS metadata come from Coolify env vars. VITE_API_URL empty
means same-origin, so the browser hits /api/todos on the same host that
serves the SPA.

frontend/src/lib/api.ts

@@ -1,7 +1,6 @@
 import { supabase } from './supabase';
 
-const API_URL = import.meta.env.VITE_API_URL;
-if (!API_URL) throw new Error('Missing VITE_API_URL');
+const API_URL = import.meta.env.VITE_API_URL ?? '';
 
 export type Todo = {
   id: number;