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/Models/Post.php

@@ -0,0 +1,57 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Support\Facades\Storage;
+
+class Post extends Model
+{
+    protected $fillable = [
+        'title',
+        'slug',
+        'excerpt',
+        'body',
+        'cover_image',
+        'author_id',
+        'published_at',
+    ];
+
+    protected $casts = [
+        'published_at' => 'datetime',
+    ];
+
+    public function getRouteKeyName(): string
+    {
+        return 'slug';
+    }
+
+    public function author(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'author_id');
+    }
+
+    public function scopePublished(Builder $query): Builder
+    {
+        return $query->whereNotNull('published_at')->where('published_at', '<=', now());
+    }
+
+    public function getIsPublishedAttribute(): bool
+    {
+        return $this->published_at !== null && $this->published_at->lte(now());
+    }
+
+    public function getCoverUrlAttribute(): ?string
+    {
+        $cover = (string) ($this->attributes['cover_image'] ?? '');
+        if ($cover === '') {
+            return null;
+        }
+        if (preg_match('/^https?:\/\//i', $cover)) {
+            return $cover;
+        }
+        return Storage::url($cover);
+    }
+}