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>

tests/Feature/ExampleTest.php

@@ -2,11 +2,13 @@
 
 namespace Tests\Feature;
 
-// use Illuminate\Foundation\Testing\RefreshDatabase;
+use Illuminate\Foundation\Testing\RefreshDatabase;
 use Tests\TestCase;
 
 class ExampleTest extends TestCase
 {
+    use RefreshDatabase;
+
     /**
      * A basic test example.
      */