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/scheduler.py

@@ -5,6 +5,7 @@ from apscheduler.triggers.interval import IntervalTrigger
 
 from app.mail_processor import process_mailbox
 from app.smb_processor import process_smb_share
+from app.ftp_processor import process_ftp
 from app.amazon_processor import process_amazon
 from app.amazon_api import process_amazon_api
 from app.database import get_settings
@@ -33,6 +34,10 @@ async def _run_processor():
         smb_result = await process_smb_share()
         logger.info(f"SMB-Verarbeitung abgeschlossen: {smb_result}")
 
+        logger.info("Starte automatische FTP-Verarbeitung...")
+        ftp_result = await process_ftp()
+        logger.info(f"FTP-Verarbeitung abgeschlossen: {ftp_result}")
+
         # Amazon separately with timeout - must not block next scheduler runs
         logger.info("Starte automatische Amazon-Verarbeitung...")
         try: