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>

routes/web.php

@@ -14,6 +14,8 @@ Route::prefix('admin')->middleware(['auth','admin'])->group(function(){
     Route::get('/', [\App\Http\Controllers\AdminDashboardController::class, 'index'])->name('admin.dashboard');
     // Admin Exercises CRUD
     Route::resource('exercises', \App\Http\Controllers\Admin\ExerciseController::class)->except(['show'])->names('admin.exercises');
+    // Admin Posts CRUD
+    Route::resource('posts', \App\Http\Controllers\Admin\PostController::class)->except(['show'])->names('admin.posts');
 });
 
 Route::middleware('auth')->group(function () {
@@ -60,6 +62,7 @@ Route::middleware('auth')->group(function () {
 
 Route::prefix('/blog')->group(function () {
     Route::get('/', [\App\Http\Controllers\BlogController::class, 'index'])->name('blog.index');
+    Route::get('/{post:slug}', [\App\Http\Controllers\BlogController::class, 'show'])->name('blog.show');
 });
 
 // Yoga poses