Add reMarkable backup system with web UI

- Docker container with rmapi cloud backup, PDF conversion pipeline
- Web UI: file browser, multi-select, delete/move, thumbnail preview
- Sync: backup from reMarkable cloud, rmdoc→PDF conversion via rmc+Inkscape
- Excluded-files mechanism to prevent deleted items from returning after sync

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

remarkable/web/auth.php

@@ -0,0 +1,26 @@
+<?php
+declare(strict_types=1);
+
+$user = getenv('RM_USER') ?: 'admin';
+$pass = getenv('RM_PASS') ?: '';
+
+if ($pass === '') return; // kein Passwort gesetzt → kein Schutz
+
+// PHP Built-in Server liefert keine PHP_AUTH_* Variablen — Header manuell parsen
+$authUser = $_SERVER['PHP_AUTH_USER'] ?? '';
+$authPass = $_SERVER['PHP_AUTH_PW']   ?? '';
+
+if ($authUser === '' && isset($_SERVER['HTTP_AUTHORIZATION'])) {
+    $decoded = base64_decode(substr($_SERVER['HTTP_AUTHORIZATION'], 6));
+    [$authUser, $authPass] = explode(':', $decoded, 2) + ['', ''];
+}
+
+$ok = $authUser === $user
+    && hash_equals(hash('sha256', $pass), hash('sha256', $authPass));
+
+if (!$ok) {
+    header('WWW-Authenticate: Basic realm="reMarkable"');
+    header('HTTP/1.1 401 Unauthorized');
+    echo 'Zugriff verweigert.';
+    exit;
+}