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/AdminDashboardController.php

@@ -3,6 +3,7 @@
 namespace App\Http\Controllers;
 
 use App\Models\Exercise;
+use App\Models\Post;
 use App\Models\User;
 use App\Models\Workout;
 use App\Models\WorkoutSession;
@@ -19,6 +20,7 @@ class AdminDashboardController extends Controller
             'sessions' => WorkoutSession::count(),
             'exercises' => Exercise::count(),
             'yogaPoses' => YogaPose::count(),
+            'posts' => Post::count(),
         ];
 
         $recentUsers = User::latest()->limit(5)->get(['id','name','email','created_at']);