feat: User-Management mit Token-basierter Authentifizierung

Backend:
- UserRecord Model + Users-Tabelle in DB
- AuthService: SHA256-Passwort-Hashing, Token-Verwaltung (ConcurrentDictionary)
- AuthMiddleware: Schützt alle /api/-Routes (außer /api/auth/login)
- AuthEndpoint: Login, Logout, Me
- Default-User admin/admin wird automatisch erstellt

Frontend:
- Login-Page mit Formular (Enter-Key-Support)
- Globaler Fetch-Interceptor: 401 → automatisch Login-Redirect
- Auth-Check beim App-Start und bei Hash-Änderungen
- Logout-Link im Dashboard-Header
- Login-CSS: Zentrierte Login-Box

Middleware/AuthMiddleware.cs

@@ -0,0 +1,40 @@
+using System.Threading.Tasks;
+using Gott.Services;
+using Microsoft.AspNetCore.Http;
+
+namespace Gott.Middleware;
+
+public class AuthMiddleware
+{
+    private readonly RequestDelegate _next;
+
+    public AuthMiddleware(RequestDelegate next)
+    {
+        _next = next;
+    }
+
+    public async Task InvokeAsync(HttpContext ctx, AuthService auth)
+    {
+        var path = ctx.Request.Path.Value ?? "";
+
+        // Static files and login endpoint don't require auth
+        if (!path.StartsWith("/api/") || path == "/api/auth/login")
+        {
+            await _next(ctx);
+            return;
+        }
+
+        var token = ctx.Request.Cookies["gott_token"];
+        var userId = auth.ValidateToken(token);
+
+        if (userId == null)
+        {
+            ctx.Response.StatusCode = 401;
+            await ctx.Response.WriteAsJsonAsync(new { error = "Not authenticated" });
+            return;
+        }
+
+        ctx.Items["UserId"] = userId;
+        await _next(ctx);
+    }
+}