Add FTP/SFTP support and tree-view folder picker with lazy loading

FTP/SFTP processor:
- New ftp_processor.py with adapter pattern for FTP (passive) and SFTP
- Same design as smb_processor: read PDFs, forward via SMTP, move to processed
- Eingangs-/Ausgangsbelege with separate paths, modes (forward/separator)
- paramiko==3.5.0 for SFTP support
- Schema v9 with new ftp_* settings
- Integrated in scheduler

Tree-view folder picker (SMB + FTP):
- Reusable tree rendering from flat path lists
- Expandable/collapsible nodes with toggle arrows
- Lazy loading: only top-level folders on open, sub-folders on-demand
- Auto-expand ancestors of currently selected value (with preload)
- Reload button stays for manual refresh
- Always fresh load when opening picker
- New endpoints: /api/list-smb-subfolders, /api/list-ftp-subfolders

FTP-specific fixes:
- list_pdfs uses LIST instead of NLST (more reliable across servers)
- Stateful CWD bug fixed in ensure_dir/stat_exists/rename
  (previously created /Buch/Buch/X instead of /Buch/X due to CWD drift)
- All operations reset CWD via _reset_cwd() before stateful calls
- _resolve() helper for SFTP to handle empty path / chroot users

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/static/style.css

@@ -424,6 +424,35 @@ small.text-muted {
     align-items: center;
 }
 
+/* Tree-View Toggle */
+.folder-tree-toggle {
+    flex-shrink: 0;
+    background: none;
+    border: none;
+    cursor: pointer;
+    width: 1.5rem;
+    text-align: center;
+    color: var(--text-muted);
+    font-size: 0.75rem;
+    padding: 0.5rem 0;
+    user-select: none;
+}
+
+.folder-tree-toggle:hover {
+    color: var(--primary);
+}
+
+.folder-tree-toggle.empty {
+    cursor: default;
+    visibility: hidden;
+}
+
+.folder-tree-children {
+    margin-left: 1.5rem;
+    border-left: 1px dashed var(--border);
+    padding-left: 0.25rem;
+}
+
 .folder-item {
     display: flex;
     align-items: center;