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>

config/session.php

@@ -8,13 +8,19 @@ use Illuminate\Support\Str;
 |--------------------------------------------------------------------------
 |
 | Kept from earlier releases, where it differs from the framework's
-| "<app>_session": renaming the cookie would sign everyone out. Every
-| other key comes from the framework's defaults.
+| "<app>_session": renaming the cookie would sign everyone out.
+|
+| Sessions are encrypted unless SESSION_ENCRYPT=false: they hold the keys
+| of the password shares a recipient unlocked, which must never sit
+| readable in the sessions table. Every other key comes from the
+| framework's defaults.
 |
 */
 
 return [
 
+    'encrypt' => env('SESSION_ENCRYPT', true),
+
     'cookie' => env(
         'SESSION_COOKIE',
         Str::slug((string) env('APP_NAME', 'laravel')).'-session'