Add authentication, a public landing page, and guest data preview

Hand-rolled login/register/logout on Laravel's built-in Auth facade
rather than Breeze/Jetstream, which scaffold Tailwind v3 config and
would fight our existing Tailwind v4 CSS-first setup. Login/Register
are Livewire components; logout is a plain POST route (auth-gated,
CSRF-protected).

Routing: "/" is now a public landing page describing the project;
the sites list moved from "/" to "/sites" to make room for it.

Data gating: App\Support\GuestPreview::paginate() forces page 1 and a
5-row page size for unauthenticated visitors on Sites, Discover, and
a site's Pages/Backlinks tables, while total() still reflects the
real match count — authenticated users get the existing full
pagination unchanged. A shared guest-preview-notice component shows
"N of M results, sign in to see all" and only renders when something
is actually being hidden.

Testing: pointed the test suite at a dedicated MySQL database
(huginn_testing) instead of the default in-memory SQLite, since the
existing fulltext index migration isn't SQLite-compatible — this app
is MySQL-specific by now regardless. Added AuthFlowTest (register,
login, wrong-password rejection, logout) and GuestPreviewTest (guest
capped to a preview, authenticated sees everything), all passing.

Also fixed a real bug the tests caught: request()->session() isn't
reliably attached during Livewire's in-process test action calls;
switched Login/Register to the session() helper, which resolves the
session store directly and works in both contexts.

app/Livewire/Auth/Login.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace App\Livewire\Auth;
+
+use Illuminate\Contracts\View\View;
+use Illuminate\Support\Facades\Auth;
+use Illuminate\Validation\ValidationException;
+use Livewire\Attributes\Layout;
+use Livewire\Component;
+
+#[Layout('components.layouts.app')]
+class Login extends Component
+{
+    public string $email = '';
+
+    public string $password = '';
+
+    public bool $remember = false;
+
+    public function login(): void
+    {
+        $this->validate([
+            'email' => ['required', 'email'],
+            'password' => ['required'],
+        ]);
+
+        if (! Auth::attempt(['email' => $this->email, 'password' => $this->password], $this->remember)) {
+            throw ValidationException::withMessages([
+                'email' => 'These credentials do not match our records.',
+            ]);
+        }
+
+        session()->regenerate();
+
+        $this->redirect(route('sites.index'), navigate: false);
+    }
+
+    public function render(): View
+    {
+        return view('livewire.auth.login');
+    }
+}