feat: HL tenant setup + email domain auto-assignment

- Create pre-configured Hogan Lovells tenant with demo flag and
  auto_assign_domains: ["hoganlovells.com"]
- Add POST /api/tenants/auto-assign endpoint: checks email domain
  against tenant settings, auto-assigns user as associate if match
- Add AutoAssignByDomain to TenantService
- Update registration flow: after signup, check auto-assign before
  showing tenant creation form. Skip tenant creation if auto-assigned.
- Add DemoBanner component shown when tenant.settings.demo is true
- Extend GET /api/me to return is_demo flag from tenant settings

frontend/src/app/(auth)/register/page.tsx

@@ -5,12 +5,22 @@ import { api } from "@/lib/api";
 import { useRouter } from "next/navigation";
 import { useState } from "react";
 
+interface AutoAssignResponse {
+  assigned: boolean;
+  tenant_id?: string;
+  name?: string;
+  slug?: string;
+  role?: string;
+  settings?: Record<string, unknown>;
+}
+
 export default function RegisterPage() {
   const [email, setEmail] = useState("");
   const [password, setPassword] = useState("");
   const [firmName, setFirmName] = useState("");
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState<string | null>(null);
+  const [showFirmName, setShowFirmName] = useState(true);
   const router = useRouter();
   const supabase = createClient();
 
@@ -34,8 +44,30 @@ export default function RegisterPage() {
       return;
     }
 
-    // 2. Create tenant via backend (the backend adds the user as owner)
     if (data.session) {
+      // 2. Check if email domain matches an existing tenant for auto-assignment
+      try {
+        const result = await api.post<AutoAssignResponse>("/tenants/auto-assign", { email });
+        if (result.assigned && result.tenant_id) {
+          // Auto-assigned — store tenant and go to dashboard
+          localStorage.setItem("kanzlai_tenant_id", result.tenant_id);
+          router.push("/");
+          router.refresh();
+          return;
+        }
+      } catch {
+        // Auto-assign failed — fall through to manual tenant creation
+      }
+
+      // 3. No auto-assignment — create tenant manually
+      if (!firmName) {
+        // Show firm name field if not yet visible
+        setShowFirmName(true);
+        setError("Bitte geben Sie einen Kanzleinamen ein");
+        setLoading(false);
+        return;
+      }
+
       try {
         await api.post("/tenants", { name: firmName });
       } catch (err: unknown) {
@@ -68,23 +100,27 @@ export default function RegisterPage() {
         </div>
 
         <form onSubmit={handleRegister} className="space-y-4">
-          <div>
-            <label
-              htmlFor="firm"
-              className="block text-sm font-medium text-neutral-700"
-            >
-              Kanzleiname
-            </label>
-            <input
-              id="firm"
-              type="text"
-              value={firmName}
-              onChange={(e) => setFirmName(e.target.value)}
-              required
-              className="mt-1 block w-full rounded-md border border-neutral-300 px-3 py-2 text-sm placeholder-neutral-400 focus:border-neutral-900 focus:outline-none focus:ring-1 focus:ring-neutral-900"
-              placeholder="Muster & Partner Rechtsanwaelte"
-            />
-          </div>
+          {showFirmName && (
+            <div>
+              <label
+                htmlFor="firm"
+                className="block text-sm font-medium text-neutral-700"
+              >
+                Kanzleiname
+              </label>
+              <input
+                id="firm"
+                type="text"
+                value={firmName}
+                onChange={(e) => setFirmName(e.target.value)}
+                className="mt-1 block w-full rounded-md border border-neutral-300 px-3 py-2 text-sm placeholder-neutral-400 focus:border-neutral-900 focus:outline-none focus:ring-1 focus:ring-neutral-900"
+                placeholder="Muster & Partner Rechtsanwaelte"
+              />
+              <p className="mt-1 text-xs text-neutral-400">
+                Leer lassen, falls Sie zu einer bestehenden Kanzlei eingeladen wurden
+              </p>
+            </div>
+          )}
 
           <div>
             <label