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>

database/seeders/DatabaseSeeder.php

@@ -15,9 +15,13 @@ class DatabaseSeeder extends Seeder
     {
         // User::factory(10)->create();
 
-        User::factory()->create([
+        $admin = User::factory()->create([
             'name' => 'Test User',
             'email' => 'test@example.com',
         ]);
+        $admin->is_admin = true;
+        $admin->save();
+
+        $this->call(PostSeeder::class);
     }
 }