Add Docker setup and fix proxy config for containerized deployment

- Program.cs: remove UseHsts/UseHttpsRedirection (NPM terminates TLS,
  would loop behind the proxy, plan.md AE-4), add UseForwardedHeaders
  with emptied known networks/proxies. Uses KnownIPNetworks (net10).
- Dockerfile: SDK build stage, chiseled aspnet runtime on port 8080.
- .dockerignore: keep build output, git and docs out of the image.
- compose.yaml: temporary test setup (builds from source, exposes
  5000:8080) for the first run on Unraid without NPM or registry.
- .gitignore: exclude .idea/ (Rider project files).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Elternbeirat.Web/Program.cs

@@ -1,4 +1,5 @@
 using Elternbeirat.Web.Components;
+using Microsoft.AspNetCore.HttpOverrides;
 
 var builder = WebApplication.CreateBuilder(args);
 
@@ -7,20 +8,32 @@ builder.Services.AddRazorComponents();
 
 var app = builder.Build();
 
+// NPM terminiert TLS und ist der einzige Weg zum Container (kein Port-Mapping
+// im Produktivbetrieb, siehe plan.md AE-4). Ohne UseForwardedHeaders sieht die
+// App jede Anfrage als HTTP und mit der Proxy-IP statt der Client-IP.
+// KnownNetworks/KnownProxies bewusst geleert, weil ausschliesslich NPM den
+// Container erreicht.
+app.UseForwardedHeaders(new ForwardedHeadersOptions
+{
+    ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto,
+    KnownIPNetworks = { },
+    KnownProxies = { }
+});
+
 // Configure the HTTP request pipeline.
 if (!app.Environment.IsDevelopment())
 {
     app.UseExceptionHandler("/Error", createScopeForErrors: true);
-    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
-    app.UseHsts();
+    // Kein UseHsts() und kein UseHttpsRedirection(): NPM setzt HSTS und
+    // terminiert TLS. Beides hier wuerde hinter dem Proxy eine
+    // Redirect-Schleife erzeugen (plan.md AE-4).
 }
 
 app.UseStatusCodePagesWithReExecute("/not-found", createScopeForStatusCodePages: true);
-app.UseHttpsRedirection();
 
 app.UseAntiforgery();
 
 app.MapStaticAssets();
 app.MapRazorComponents<App>();
 
-app.Run();
+app.Run();