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>

lang/en/app.php

@@ -246,6 +246,7 @@ return [
             'sessions' => 'Sessions',
             'exercises' => 'Exercises',
             'yoga_poses' => 'Yoga poses',
+            'posts' => 'Blog posts',
         ],
         'recent_users' => 'Recent users',
         'recent_workouts' => 'Recent workouts',