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>

docker-compose.example.yml

@@ -4,12 +4,13 @@
 #
 # Quick start:
 #   1. Copy this file:  cp docker-compose.example.yml docker-compose.yml
-#   2. Edit the settings below (APP_URL is required; uploads need HTTPS, see below)
+#   2. Edit the settings below (APP_URL and APP_KEY are required; uploads need HTTPS, see below)
 #   3. Start:  docker compose up -d
 #   4. Open your browser to your configured domain
 #
-# Note: APP_KEY is auto-generated on first start if not set.
-#       Copy it from the logs into your docker-compose.yml to persist across restarts.
+# APP_KEY is required: the container does not start without it. Create one with
+#   docker run --rm --entrypoint php gitea.nonameweb.ch/nonameweb/sealshare:latest artisan key:generate --show
+# and keep it: it decrypts the shares, so a new key makes the existing ones unreadable.
 #
 # ============================================
 
@@ -32,7 +33,7 @@ services:
     environment:
       # --- REQUIRED ---
       APP_URL:              # Your full URL, e.g. https://share.example.com
-      # APP_KEY:            # Auto-generated if not set. Copy from logs to persist across restarts.
+      APP_KEY:              # Required. Create one with php artisan key:generate --show (see above)
       DB_DATABASE: /app/database/sqlite/database.sqlite  # The SQLite file in sealshare_database
 
       # --- HTTPS ---
@@ -90,7 +91,7 @@ services:
       - sealshare_storage:/app/storage/app
       - sealshare_database:/app/database/sqlite
     environment:
-      # APP_KEY:            # Same key as the app service above (auto-generated if not set)
+      APP_KEY:              # Same key as the app service above
       APP_URL:              # Same URL as the app service above
       DB_DATABASE: /app/database/sqlite/database.sqlite  # Same as the app service above
     depends_on: