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/sync.php

@@ -0,0 +1,64 @@
+<?php
+declare(strict_types=1);
+require __DIR__ . '/auth.php';
+
+$BASE_DIR      = rtrim(getenv('BASE_DIR') ?: '/data', '/');
+$BACKUP_SCRIPT = '/usr/local/bin/remarkable-backup.sh';
+$CONVERT_SCRIPT= '/usr/local/bin/remarkable-convert.sh';
+$SNAPSHOT_KEEP = getenv('SNAPSHOT_KEEP') ?: '60';
+
+// Läuft bereits ein Sync?
+$LOCK = $BASE_DIR . '/.backup.lock';
+
+header('Content-Type: text/plain; charset=utf-8');
+header('X-Accel-Buffering: no');
+header('Cache-Control: no-cache');
+if (ob_get_level()) ob_end_clean();
+
+function stream(string $cmd): int {
+    $handle = popen($cmd . ' 2>&1', 'r');
+    if ($handle === false) return 1;
+    while (!feof($handle)) {
+        $chunk = fread($handle, 256);
+        if ($chunk !== false && $chunk !== '') {
+            echo $chunk;
+            flush();
+        }
+    }
+    return pclose($handle);
+}
+
+echo "=== Backup ===\n";
+flush();
+$exit = stream("SNAPSHOT_KEEP={$SNAPSHOT_KEEP} {$BACKUP_SCRIPT} {$BASE_DIR} /");
+
+if ($exit !== 0) {
+    echo "\n[FEHLER] Backup fehlgeschlagen (exit {$exit})\n";
+    exit;
+}
+
+// Web-gelöschte Dateien wieder entfernen (kommen durch Sync zurück)
+$excludeFile = $BASE_DIR . '/.web_deleted';
+if (file_exists($excludeFile)) {
+    $lines = file($excludeFile, FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
+    $removed = 0;
+    foreach (array_unique($lines) as $p) {
+        $p = '/' . ltrim($p, '/');
+        $local = $BASE_DIR . '/current' . $p;
+        if (is_dir($local)) {
+            shell_exec('rm -rf ' . escapeshellarg($local));
+            $removed++;
+        } else {
+            foreach ([$local, $local.'.pdf', $local.'.rmdoc', $local.'.thumb.jpg'] as $f) {
+                if (is_file($f) && @unlink($f)) $removed++;
+            }
+        }
+    }
+    if ($removed) echo "\n[Web-Löschung] $removed Element(e) erneut entfernt.\n";
+}
+
+echo "\n=== PDF-Konvertierung ===\n";
+flush();
+stream("{$CONVERT_SCRIPT} {$BASE_DIR}/current");
+
+echo "\n=== Fertig ===\n";