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.

.dockerignore

@@ -0,0 +1,28 @@
+**/bin/
+**/obj/
+**/node_modules/
+**/dist/
+**/.vite/
+
+.git/
+.gitignore
+.vs/
+.idea/
+.vscode/
+
+*.user
+*.swp
+
+**/appsettings.Development.json
+**/appsettings.Local.json
+.env
+.env.local
+frontend/.env
+frontend/.env.local
+
+Dockerfile
+docker-compose.yml
+.dockerignore
+README.md
+docs/
+migrations/