Add blog feature and fix broken test/build infra

BlogController was an empty stub with no view, so /blog 500'd on every
visit. Adds a real Post model, public listing/detail pages, and an admin
CRUD screen mirroring the existing Exercise admin pattern (image upload
or URL, auto-slug, publish toggle). Seeds 5 real posts so it's not empty
on launch.

Also fixes two things that were silently broken: phpunit.xml pointed
DB_DATABASE at a non-existent "testing" file with no DB_CONNECTION set,
and the stock ExampleTest had RefreshDatabase commented out - both meant
the test suite never actually ran successfully.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/Http/Controllers/BlogController.php

@@ -2,9 +2,48 @@
 
 namespace App\Http\Controllers;
 
+use App\Models\Post;
 use Illuminate\Http\Request;
+use Illuminate\View\View;
 
 class BlogController extends Controller
 {
-    //
+    public function index(Request $request): View
+    {
+        $q = trim((string) $request->query('q', ''));
+
+        $posts = Post::query()
+            ->published()
+            ->with('author')
+            ->when($q !== '', function ($qb) use ($q) {
+                $qb->where(function ($sub) use ($q) {
+                    $sub->where('title', 'like', "%{$q}%")
+                        ->orWhere('excerpt', 'like', "%{$q}%");
+                });
+            })
+            ->latest('published_at')
+            ->paginate(9)
+            ->withQueryString();
+
+        return view('blog.index', [
+            'posts' => $posts,
+            'filters' => ['q' => $q],
+        ]);
+    }
+
+    public function show(Post $post): View
+    {
+        abort_unless($post->is_published, 404);
+
+        $post->load('author');
+
+        $relatedPosts = Post::query()
+            ->published()
+            ->where('id', '!=', $post->id)
+            ->latest('published_at')
+            ->limit(3)
+            ->get();
+
+        return view('blog.show', compact('post', 'relatedPosts'));
+    }
 }