Fix the security audit's findings

ShareDownload::$authenticated was a public property without #[Locked],
so a visitor with a share's link but not its password could set it from
the browser and see the file list: names, folder paths, sizes and
whether the share holds a text. The contents stayed protected; the flag
is now locked, and a test tries to set it.

Sessions hold the keys of the password shares a recipient unlocked, and
were stored unencrypted in the sessions table wherever SESSION_ENCRYPT
was not set, which includes every Docker install. They are now
encrypted unless SESSION_ENCRYPT=false. Share keys never live in the
session, so every share stays readable; people sign in again once.

The container generated an APP_KEY when none was set, printed it to the
logs and generated another on the next start, which left the shares
without a password unreadable. It now refuses to start without one and
says how to create it; docker-compose.example.yml requires it for both
services. The command it names, and the README's, now pass
--entrypoint php: the image's entrypoint ignores its arguments, so
"docker run <image> php artisan key:generate --show" never ran it.

Every proxy is trusted, so the password-reset mail built its link from
a host a forged X-Forwarded-Host could choose. It is now built from
APP_URL.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/Feature/SecurityAuditTest.php

@@ -10,6 +10,7 @@ use App\Services\ShareService;
 use Illuminate\Http\UploadedFile;
 use Illuminate\Support\Facades\RateLimiter;
 use Illuminate\Support\Facades\Storage;
+use Livewire\Features\SupportLockedProperties\CannotUpdateLockedPropertyException;
 use Livewire\Livewire;
 
 // --- Session stores derived key, not plaintext password ---
@@ -35,6 +36,22 @@ test('session stores derived encryption key instead of plaintext password', func
     expect(strlen(session('share_key_'.$share->token)))->toBe(64);
 });
 
+// --- The password gate cannot be opened from the browser ---
+
+test('a visitor without the password cannot unlock a share\'s file list from the browser', function () {
+    Storage::fake('shares');
+
+    $share = app(ShareService::class)->createShare([
+        ['file' => UploadedFile::fake()->createWithContent('salary-review.pdf', 'secret'), 'relativePath' => null],
+    ], [
+        'password' => 'test-password-secure',
+    ]);
+
+    Livewire::test(ShareDownload::class, ['share' => $share])
+        ->assertDontSee('salary-review.pdf')
+        ->set('authenticated', true);
+})->throws(CannotUpdateLockedPropertyException::class);
+
 // --- Rate limiting on password verification ---
 
 test('rate limiting blocks after 5 failed password attempts', function () {