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/factories/PostFactory.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Database\Factories;
+
+use App\Models\User;
+use Illuminate\Database\Eloquent\Factories\Factory;
+use Illuminate\Support\Str;
+
+/**
+ * @extends \Illuminate\Database\Eloquent\Factories\Factory<\App\Models\Post>
+ */
+class PostFactory extends Factory
+{
+    public function definition(): array
+    {
+        $title = rtrim(fake()->sentence(rand(4, 8)), '.');
+
+        return [
+            'title' => $title,
+            'slug' => Str::slug($title) . '-' . fake()->unique()->numberBetween(1, 100000),
+            'excerpt' => fake()->sentence(rand(15, 25)),
+            'body' => implode("\n\n", fake()->paragraphs(rand(4, 7))),
+            'cover_image' => null,
+            'author_id' => User::query()->inRandomOrder()->value('id'),
+            'published_at' => now()->subDays(fake()->numberBetween(0, 60)),
+        ];
+    }
+
+    public function draft(): static
+    {
+        return $this->state(fn (array $attributes) => [
+            'published_at' => null,
+        ]);
+    }
+}