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/smb_processor.py

@@ -99,7 +99,7 @@ def _move_smb_file(source: str, dest_dir: str, filename: str):
 
 
 def _list_smb_folders_recursive(
-    base_path: str, max_depth: int = 3, _current_depth: int = 0, _prefix: str = ""
+    base_path: str, max_depth: int = 5, _current_depth: int = 0, _prefix: str = ""
 ) -> list[str]:
     """Recursively list folders on the SMB share, returning relative paths."""
     folders = []
@@ -307,7 +307,7 @@ async def process_smb_share() -> dict:
 
 
 async def test_smb_connection() -> dict:
-    """Test SMB connection and return folder list."""
+    """Test SMB connection and return TOP-LEVEL folders only (lazy loading)."""
     settings = await get_settings()
 
     if not settings.get("smb_server") or not settings.get("smb_share"):
@@ -315,7 +315,7 @@ async def test_smb_connection() -> dict:
 
     try:
         base_path = await asyncio.to_thread(_smb_register_session, settings)
-        folders = await asyncio.to_thread(_list_smb_folders_recursive, base_path, 3)
+        folders = await asyncio.to_thread(_list_smb_folders_recursive, base_path, 1)
         return {"success": True, "folders": sorted(folders)}
 
     except Exception as e:
@@ -347,13 +347,34 @@ async def create_smb_folder(folder_path: str) -> dict:
 
 
 async def list_smb_folders() -> dict:
-    """Return current folder list from SMB share."""
+    """Return TOP-LEVEL folder list from SMB share (lazy loading)."""
     settings = await get_settings()
     if not settings.get("smb_server") or not settings.get("smb_share"):
         return {"folders": []}
     try:
         base_path = await asyncio.to_thread(_smb_register_session, settings)
-        folders = await asyncio.to_thread(_list_smb_folders_recursive, base_path, 3)
+        folders = await asyncio.to_thread(_list_smb_folders_recursive, base_path, 1)
         return {"folders": sorted(folders)}
     except Exception:
         return {"folders": []}
+
+
+async def list_smb_subfolders(parent_path: str) -> dict:
+    """List direct subfolders of a path (one level deep, for lazy tree expansion)."""
+    settings = await get_settings()
+    if not settings.get("smb_server") or not settings.get("smb_share"):
+        return {"success": False, "error": "SMB nicht konfiguriert", "folders": []}
+    try:
+        base_path = await asyncio.to_thread(_smb_register_session, settings)
+        full_path = _smb_unc_path(base_path, parent_path) if parent_path else base_path
+        # max_depth=1 returns only direct children
+        rel_folders = await asyncio.to_thread(_list_smb_folders_recursive, full_path, 1)
+        # Prefix with parent_path so the frontend has full paths
+        if parent_path:
+            folders = [f"{parent_path}/{f}" for f in rel_folders]
+        else:
+            folders = rel_folders
+        return {"success": True, "folders": sorted(folders)}
+    except Exception as e:
+        logger.error(f"SMB-Subfolder-Liste fehlgeschlagen: {e}")
+        return {"success": False, "error": str(e), "folders": []}