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.

backend/Program.cs

@@ -49,9 +49,14 @@ if (app.Environment.IsDevelopment())
     app.UseCors(DevCorsPolicy);
 }
 
+app.UseDefaultFiles();
+app.UseStaticFiles();
+
 app.UseAuthentication();
 app.UseAuthorization();
 
 app.MapControllers();
+app.MapGet("/health", () => Results.Ok(new { status = "ok" }));
+app.MapFallbackToFile("index.html");
 
 app.Run();